PIPENET News Spring 2013
Contents
1. V V N V d A V Y X V V Oil Tank A 7 N lt A V _ d i A gt Accumulators A replace end pipe These pseudo accumulators will have the same dimensions as the pipe that they have replaced a Label Input node Type Diameter Length Reference Fluid D Reference gas Heat capacity ratio Reference Gas Te Reference Gas Pr Results selected This allows us to calculate a more accurate maximum pressure In this case it turns out that the most remote nozzles are in fact those at the top and bottom branches of the deluge system looking at the pressure extrema summary data in the output Maximum pressure is 122 052 bar G on pipe 163 at the outlet at time 10 88200 seconds PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 The graphical output shows the following Undamped Pressure Surge caused by Deluge Priming 135 Pressure Surge 120 reaches 122 05 Bar G mmm Second Peak from other branch at around 112 Bar G 105 E Ea 15 Terror eee peer ep errr perrrprerrprerry E EEE Oe Oe Oe Oe Oe Oe Oe Oe ee ee ee ee ee aa Tt Tere pe rer DRE AGE aa wrt wr ee ee 0 0 5 1 1 5 2 2 5 3 35 4 45 5 5 5 6 6 5 7 7 5 8 8 5 9 95 10 105 11 115 12 125 13 13 5 14 145 15 15 5 16 165 17 175 18 185 19 195 20 Time second E Inlet pressure of nozzle 35 E Inlet pressure of nozzle 21 These are clearly unacceptable pressure surges so something must be done Building a Simple2. convergence For a real case this process would be iterated several more times to find the optimal solution as well as looking for suitable buffers in case of unforeseen circumstances as will be covered in the next part For now it has been determined that the optimal parameters for our controller are a Gain of 0 075 Bar and a Reset Time of 1s The Effect of Transient Events So far we have only looked at a single transient event that being a 5s increase in the pressure from 15 Bar G to 20 Bar G which could be associated with the switching on of a pump or some other normal operating procedure Assuming now that something different occurred such as a 10 Bar G pressure increase due to the system cutting off the supply to other consumers while maintaining the pump speed meaning that rather than increasing the pressure from 15 Bar G to 20 Bar G it now went to 25 Bar G Would the network still be stable Below can be seen the results from the same network but with the higher pressure changing to 25 Bar G in the same amount of time Effect of increasing pressure by 5 Bar Considerable instability 0 65 5 E Outlet pressure of valve 1 E Setting of valve E Outlet pressure of valve 1 low This has clearly shown that in this case our control system is not suitable causing considerable instability Looking at our previous results we need to see what we can do to mitigate this instability The dominant term is the Gain so this
3. 0 2s The results are seen below Linear Valve Closure with unstable parameters 9 307 r0 92 9 27 70 88 70 84 9 245 70 80 9 215 70 76 9 18 0 72 9 154 rs 0 68 5 3 2 S T912 Large amount of instability 3 aie F a 5 50 64 gt 9 094 9 065 9 035 9 00 8 97 8 947 a rvv ri a CC CE CE E E REL PE II E O CR E CR E E E EL E E E OR RR E E RI RI ARI EA AI CJ E E SRR CI RI RL DR E RL E RC RE E E RL LI CR E ZI RE EL LR RI ee ee EL o 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Time seconds E Outlet p f valve 1 E Setting of valve 1 E Outlet pressure of valve 1 low Changing to a first order valve closure profile with a 1 second time constant PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Changing to first order closure profile Low Peak Rapid valve closure Very Stable 0 44 II T T 7 ro 7 ro 7 T T T T T T T T 1 7 1 7 T T m rr T rr T r0 40 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 This clearly demonstrates the benefit of this type of valve closure profile although this will be determined by the type of valve chosen This is because due to the smoother manner of valve closure the response of the PID Control system can be faster by using higher gain and smaller reset time as it does not have to worry about controlling the valve closure speed Summary PID Controller tuning is a step by step process of improvement and re calculation and this is modelled in
4. Accumulator near the Most Remote Nozzles A cheap and simple solution to this problem is to extend the length of the branches by 1m beyond the end nozzles using these to act as accumulators cushioning the pressure surge In PIPENET these will be modelled as real accumulators rather than using the dry pipe model as the accumulator model is better suited to modelling the cushioning effect PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Deluge Valve 8 barg i JNA V Accumulators model the short extensions to the branches The accumulator input data is shown below Both should be the same x a Label 4 Input node 229 Type Horizontal c Diameter 50 mm Length 1 m Reference Fluid D 1 mm Reference gas Diatomic gas gt Heat capacity ratio 14 Reference Gas Te 21 oC Reference Gas Pr 0 Bar G Results selected NO The results speak for themselves Maximum pressure is 18 3877 bar G on pipe 163 at the outlet at time 11 03400 seconds The pressure surge peak has been reduced by a factor of nearly 7 and is now considerably more manageable The graphical result is shown below PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Effect of adding 1m of pipe to each branch to act as accumulators ee eee Lower Peaks F 2r wee a a EP ET PETE E rY ue oe r Tere I a GR TPT Errr rrre r I ee LS ae ae ee it peT UI a UI II en 0 05 1 1 5 2 25 3 35 4 45 5 55 6 65 7 75 8 85 9 95 10 105 11 115
5. Import graphical underlay option A dialog will open allowing the user to choose the file he wishes to import as an underlay Not the underlay format must be one of the following dxf emf and wmf F PIPENET SDF1 1 F File Init Libraries Calculation Output Colouration Edit Tools Window Help D a i 8 amp y Toolbar Display HF x gG v Status Bar N None Calculation fgg A stor Tools R et N Find dh lt Any gt Schematic window Data window Component Numb a apa TI chematic overview Information 0 Zoom E Pipe 0 View graphical underlay See le View Network Height Profile Fluid dampe 0 100 Inertial check 0 100 PEAT EE 7 Question Is it possible for me to enter a higher velocity for mm mm m sec smaller diameter pipe in the pipe types maximum velocity 400 381 6 i section 450 500 478 6 600 575 bore and all velocities must be set or no velocities currently generate the following warning when must be set It currently fails to do so at bore 600 Maximum flowrate must increase with increasing nominal try to do so PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Answer It is not possible to have a higher flow velocity for smaller pipes This is to allow the pipe sizing to work by always selecting the smallest pipe allowable for the required flow rate If you have already sized all your pipes you can split the schedule into
6. PIPENET They should always be tuned based on the worst case scenario namely that which causes the greatest pressure surge over the shortest period of time The response of PID controllers is slower but more stable with lower gain and a larger reset time hence it is recommended to start with the parameters set as such then slowly increasing the gain and decreasing the reset time until results are satisfactory In the case of differential control the rate time should be around 25 of the reset time to maximise Stability Anti Windup is also recommended for use to prevent the control system from taking a long time to reset the output value in case of overshoot As this is often specified by default in industry it is recommended that this is kept to the default value in PIPENET It is important that safety margins in the control parameters are kept if the network to be modelled has not been completed as any change in the network even one as simple as pipe length could result in an unsatisfactory control system if the margins are insufficient Using different valve closure profiles will also allow a reduction in oscillations due to the damping effect being external to the controller PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 How to Reduce Pressure Surge in a Dry Deluge System Introduction Pressure surge is a commonly found phenomenon when a dry deluge system is primed In the worst cases the maximum pressure can reach values of well
7. over 100 Bar G when the most remote nozzle is primed This occurs as the resistance to flow is increasing at which point an instant change of fluid from air to water occurs decreasing the flowrate instantly as well The following network is one which suffers from such a problem the maximum pressure peak reaching 122 05 Bar G Here we will analyse two cost efficient ways to minimise the pressure surge The first is to create a simple accumulator using a short dry pipe near the most remote nozzles and the second is to reduce the pipe sizes limiting the flowrate as the network primes Setting up the model Deluge Valve 8 barg i A VA Opens in 5s Hydraulically V Remote Nozzle nu e ee Hydraulically ZN Remote Nozzles LS The network data is as follows Inlet Pressure 8 Bar G constant throughout the simulation Deluge Valve Open Time 5 s Main Pipe Nominal Diameter 100 mm Riser Nominal Diameter 80 mm PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Branch Nominal Diameter 50 mm The pipe at the far end of the middle deluge ring should act as an accumulator when the far nozzles are primed PIPENET s dry pipe model assumes all the air is evacuated as priming occurs which means that when the pipe at the far end is filled it will result in an extra surge rather than acting in a damping manner as it should As such it is best to model this as two accumulators as follows Deluge Valve 8 barg A AY A5 V
8. still select them and change the number of decimal places to be shown You can separately choose the number of decimal places displayed for general text use dialogs etc and the number displayed when annotating or labelling the Schematic drawing You will generally want fewer on the schematic drawing to avoid clutter Question Does changing the precision affect the accuracy of the calculations Answer No It only affects the number of decimal places when the results are displayed Question Are my unit selections saved when exit PIPENET Answer The sdf file contains information about the desired units and precision So if you save the file when you exit you will automatically be saving your unit selections in the file and the next time you or someone else opens the file they will have the same unit selections PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Question Do have to do this every time create a new file Answer No When you have chosen a set of units and precisions that you like click the Save as defaults button This will save them as a personal preference in the registry and whenever you create a new sdf file it will be initialised with your saved default values of course you can change the values for your new sdf file afterwards if you want to Do note though that Transient has a different set of saved defaults to Standard and Spray Sprinkler So if you use all three modules you will need to s
9. two one with larger diameters and one with smaller diameters both of which can have different maximum velocities Both schedules can be set up with the same physical parameters in the Schedule library These can then be entered into the network and will generate warnings if these velocities are exceeded Question Which orifice plate model should choose Answer This depends on the criteria of your design Generally speaking however the BS 1042 model is normally for the measurement purposes Heriot Watt is normally for scientific research purposes and the Crane model is normally more popular for the engineering projects A more detailed explanation is given below of each orifice plate model Plates with flange tappings in accordance with BS1042 taking into account pressure recovery downstream The restrictions of BS1042 are applied so plates may only be used in pipes with diameters in the range 2 14 inches 50 8 355 6 mm Furthermore the ratio of the orifice diameter to the pipe diameter must be in the range 0 1 0 748 for pipes over 4 inches in diameter Question How do change the number of decimal places displayed Answer Using the same form select the measure unit in the left hand pane and choose the number of decimal places using the precision drop down Note this works even if you are using one of the standard unit systems like SI or Metric Don t worry that the measure and unit are greyed out you can
10. 1 InputiOutput nada YES Results selected NO Specification Type Pressure Power ramp Start time 5 Time Function sac Start value 15 Bar G Stop time 10 sec stop value 20 Bar G Exponent 1 Results Minimum pressure n a Time of min press nla Maximum pressure n a Time of max pres n a 20 19 18 17 16 15 Oo 20 40 60 80 Pressure Bar G 100 Time sec Label 4 InputOutputnode YES gt Results selected NO Specification Type Time Function Constant value Pressure gt Constant 5 Bar G PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Setting the PID Controller setting up a PID controller is an iterative process and as such many parameters can only be truly determined after a succession of test runs and may not be known at the start The key parameters in this type of calculation are the gain and the reset time Others such as set points are often known at the start and these can be input with confidence here The input set point is the control target which in this case is 9 Bar_G The output set point represents the initial output signal were the input signal the input set point in a proportional control system If this is not Known which is likely a mid range value can be selected such as 0 5 for an information output as it is in this case Gain is a far more difficult parameter to nail down as without extensive knowledge of the network its value could take a large range
11. 12 125 B 13 5 14 145 15 155 16 16 5 17 17 5 18 18 5 19 195 20 Time seconds Using Smaller Pipes to reduce the flow rate when primed Here the pipes in the network have been made smaller in order to reduce the flow rate and hence reduce the surge by increasing the pressure loss However it must be noted that the flow velocity in the pipes and the pressure at the inlet to the nozzle must be checked carefully as they still need to satisfy the original deluge system requirements The changes to the pipe sizes are as follows Main Pipe Nominal Diameter 65 mm Riser Nominal Diameter 50 mm Branch Nominal Diameter 32 mm Remote Nominal Diameter 25 mm PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Deluge Valve 8 barg 6 Branch ND 32 mm Xa Riser ND 50mm eri NE XS V Remote ND 25 mm V WON Main Pipe ND 65mm gt Branch ND A V y 32 mm eee pd A VN v V Oil Tank A 7 ma em Remote ND a A S7 25 mm PS i Pa can n nes e Branch ND 32 mm AN AZ a x LN ye A Pe ama tai Remote ND 25 mm Once more a significant reduction in surge is seen Maximum pressure is 16 9769 bar G on pipe 163 at the outlet at time 10 84400 seconds This has the following graphical output PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Effect of reducing Pipe Diameters Lower Peaks 27 wrt LE a a wee were ep ee epee ee a a ee a rrrereperrryp T n a A a a Ra T LI A wT Terre wre LE LII i UI
12. ENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 News PIPENET at OTC 2013 PIPENET will be at the Offshore Technology Conference in Houston Texas from the 6 to the 9 May 2013 in booth 11504 The offshore technology conference is the world s leading event in the fields of drilling exploration production and environmental protection in the development of offshore resources We hope to see you all there PIPENET at FireExpo 2013 except if you are attending FireExpo 2013 PIPENET will also be at the Fifteenth International Fire Protection Equipment Technology Conference and Exposition from the 7 to the 9 May at the China National Convention Center in Beijing We will be there with our Dealer Woshan Technology Beijing Co Ltd FireExpo is the largest international fire protection event for the exchange and exhibition of fire protection equipment PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Setting up a PID Control System in PIPENET step by step Introduction The design of a PID control system in PIPENET has two objectives The first is that the system can reach a steady state irrespective of the events which occur during the simulation and the second is that the system can change states smoothly and quickly However due to the interaction between events network components and control parameters it is not normally an easy task to build such control systems As such we must not consider the control system in isolation and
13. II II i Oo 05 1 13 2 25 3 ao 4 45 5 55 6 65 7 15 8 85 9 95 10 105 11 115 12 125 13 13 5 14 14 5 15 155 16 16 5 17 17 5 18 18 5 19 19 5 20 Time seconds E Inlet pressure of nozzle 35 E Inlet pressure of nozzle 21 After priming the lowest nozzle pressure on the most remote nozzle 21 is now approximately 1 95 Bar G with a minimum flow velocity of around 3 9 m s The maximum flow velocity is 6 5 m s These would have to be checked against the requirements of the deluge system as well as ensuring that noise and vibration in the pipes was not too great and may require further optimisation of pipe sizes to achieve the desired result Conclusions We have seen here two cost effective and simple ways of considerably reducing the maximum pressure in a pressure surge scenario caused by deluge system priming reducing surge by a factor of almost 7 in both cases PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Frequently Asked Questions Question Which friction factor does PIPENET use in its calculation for pressure loss due to friction Answer The friction factor used by PIPENET is the Fanning friction factor f but it is sometimes confused with the Moody friction factor fm They are related by the following fm pan Using this notation the friction can be written as below p _ 2fLpu _ fmLpu fric p 2D Question How do I import a graphical underlay Answer Open a new file in PIPENET and under View click on the
14. NEWS VOLUME 2 ISSUE 11 SPRING 2013 As an increase in gain has been successful a further increase in gain can be considered Gain 0 075 0 92 0 88 Lower Peak 710 4 0 845 710 2 0 807 0 764 710 0 0 725 198 3 e 0 68 i 2 io r s a 96 0 645 0 60 Faster Convergence P 0 567 9 2 0 524 a i Ea T Sa E 0 0 48 0447 T T T T T T T T T T tm T T T T a T T T r T r8 8 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Time seconds E Outlet pressure of valve 1 E Setting of valve 1 E Outlet pressure of valve 1 low Gain 0 1 10 47 0 92 10 34 0 88 Peak Lower Still 10 2 0 84 10 1 0 80 10 04 9 9 10 76 4 9 84 0 72 O 4 i 3 9 71 3 2 s z 0 68 F 96 S a 1 0 64 9 54 Convergence would be faster were it not for instability 947 Instability due to high gain 0 60 9 35 0 56 9 24 0 52 9 14 li 0 48 m E 9 0 WMATA AR AAR A 8 95 T 7 Tr r T 7 ro T 7 1 Tr rr r T 7 Tr r r i T r i T r 7 1 r r0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 E Outlet pressure of valve 1 E Setting of valve 1 E Outlet pressure of valve 1 low Time seconds The gain having further been increased instability can be seen As such the gain shall be kept at 0 075 Bar PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 The reset time will now be optimised in a similar manner As we
15. PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 a 4 SA ee al a _ _ a 1 News 11 al ENET A OE 8 aceia ree oa a aa e a IG la i iar eee ee 2 Lor a mel mal AF 123 d e 0 ged A pn It eee eee eee ee en er nen emer e RSI T 2 2 Setting up a PID Control System in PIPENET Step by Step JE la ugaie Ile 6 JIRI E PRR ent ERIE RE E Peay A ter ee ENE ert 3 PAE E T A AIR RER EA EA RR RR AONE A RI 3 PRSS e E eo oii e cee ene RR SERIE OREI RI PPR N RP E NR N E RR AN N N ERIS 4 a arses anti cease plan ei it aa ai ii ial elita lat 5 pe stea ale the PID 8 e laice AIE REP RRERE AES SRR ERIE MERE PERIE 9 II ier ten te ENI RE anin 7 rege e Be ali ae iii AE ep pc URR eee E RR RR OI AR RR NR ARN RR RI RE 10 2d pe z ie ee oO ranson i a IER IEI E A IRI RD E NI en ee eee RE NI JAR PRR 15 20 pie a me iti 7 de ARIEI ST SRR E IERI PTR MR RR SP PI RER ORI CINE RI 14 2 9 The Effect of Components cane ci ma a ananas a 15 3 How to Reduce Pressure Surge in a Dry Deluge System gt mae La fe 12 T RER RER RR E RR RR RI RE INI OR RONI II E RI NOROI A STARII RN NR 18 3 2 Setting up the Model mc eee eee eee eee eee na 18 33 BUNING A SIMPIE Core Dap Il e Li O EERO PRIORITARE PRR PR SR ORI PERENE NISE RR 20 3 4 Using Smaller Pipes eee eee eee ea na ee 22 a e OCIS ION iii aie bila o a a a iaca o 0 a i nt 24 4 Frequently Asked Questions 5 Contact Us PIP
16. ave your default values twice once for Transient and once for the other two Question Is there any existing library for nozzles and valves in the Spray Sprinkler module Answer There is no pre set nozzle library You can add nozzles to the system either by clicking on the library menu clicking on the nozzle option and then storing a new nozzle Local Libraries Schedules Nozzles Pumps coefficients unknown Pumps coefficients known Linings Deluge valves User Defined Nozzle Name NOZ 3 Description User Defined Nozzle Source Local user library K factorim h Bar 5 Minimum pressure Bar G 1 Maximum pressure Bar G 20 Spray divergence angle optional deg 2 ox cma or by entering the details in the property window under User defined x Y Label 1 Input node 1 Nozzle on YES gt Nozzle type User defined gt Flowrate orArea Flowrate v Requiredflowrate 22 m h K Factor 1 m h Bar Min Operating Pre 1 Bar G Max Operating Pre 20 Bar G Spray angle 2 deg Results Calc pressure n a Required flow n a Calc flow rate n a Deviation n a Flow rate fm 3 h Pressure drop across nozzle Bar PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Below is a brief description of the valves available in the Spray Sprinkler module Note these are available as separate components and some require extra user input e An overboard dump valv
17. avespeed 1379 3 m sec Results selected NO Section Type Constant Gr gt The Valve data is shown below Label 1 Input node 2 Output node 3 Input info node 5 Type Flow coef Opening Profile Linear Flow Coefficient 2000 l min Bar Results selected YES This transfer function exists for the purposes of modelling the physical characteristics of the valve In order to model the physical open and closure time of the valve the ramp up and down limits of the transfer function can be set to 0 2 s and 0 2 s respectively Furthermore the output range is limited to being between 0 and 1 as the valve cannot be open further than fully open or closed further than fully closed hence the use of limiting power ramp for the order It will have no effect on the control of the valve hence the gain is set to 1 and the bias is set to 0 O Label Input node Output node 5 Input type Information Output type Information l Order Limiting pow Gain Hias Output range min Output range max Ramp up limit 0 2 1 Isac Ramp down limit 0 2 4 sec Results selected NO PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 The pressure sensor should be set to analoque Label 1 Input node 3 Information node 6 Analogue Results selected NO Analogue Digital The inlet specification should be a power ramp from 15 Bar G to 20 Bar G set to happen in 5 seconds starting at the 5 s mark Label
18. e or pressure safety valve which operates with a trigger pressure e The deluge valve which follows the following modelling equation P Q K where P is the pressure drop Q is the flow rate K is a constant for the valve xX is a constant for the valve usually 1 or 2 e The non return valve allows unrestricted flow in the positive direction and prevents flow in the reverse direction e The elastomeric valve allows the user to achieve the required input pressure output pressure pressure drop or flow rate without the need to input valve characteristic data For a detailed description of these valves please look through the help menu Contents under Modelling Question Can add user defined fittings in the Spray Sprinkler module Answer The range of fittings in the Spray Sprinkler module is based on the guidelines of the NFPA If you wish to add some device or fitting not included in the range of available fittings then you may be able to consider the use of an equipment item which allows you to input a defined equivalent length Re OT PO OM y Dogo Ow g as CQUIDMENT a Label 1 Pipe Label 6 Description Equiv Length Unset m PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 CONTACT US Technical Support support sunrise sys com Sales pipenet sunrise sys com a gt SUNRISE
19. est to start with a small gain and then increase it step by step until the model delivers satisfactory results reaching steady operation quickly and smoothly At this point it must be determined which type of controller needs to be used This is usually known to the control system engineer but as a rule of thumb PID controllers are the most effective but the most expensive proportional controllers P controllers are the cheapest and least effective and PI controllers normally offer a balance between the two For more information on the model equations of the different types of controllers please refer to the footnote at the bottom of this article or the PIPENET User Manual found in the help menu For this network a PI controller will be used as it is the most commonly chosen type and as such the parameters that will be focused on are those that require tuning in a PI controller Before that however a brief word on differential control By introducing a differential term oscillations can be dampened faster resulting in greater stability These require the extra term known as the rate time which determines the effect of the differential term If set too large gt Reset Time 4 instabilities can occur as the differential control term takes over Returning to our Proportional Integral controller due to the integral term the reset time must be set The reset time should be tuned based on the dynamic response of the network In the g
20. etting the tracking time manually Below can be seen the initial settings for the controller Label Input node Output node T Sensor type Pressure Control type PI Input set point 9 Output set point 0 5 Gain 0 05 Reset time Anti windup YES Tracking time Default 2 5 Results selected NO The results of the initial calculation can be seen below PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Results from Initial Settings 10 8 70 88 10 64 0 84 Initial small Lo so 10 45 oscillations 0 76 10 24 0 72 10 07 o 0 68 _ 3 984 a A 5 Lo 64 Only reaches 9 9 65 Bar G Stability at around 100s mark Lo 60 9 45 0 56 9 Bar G Line 9 2 0 52 88 E CN E E ee DI r0 44 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Time d E Outlet pressure of valve 1 E Setting of valve 1 E Outlet pressure of valve 1 low While stability is good and oscillations minimal it can be seen that it takes around 100s before stabilising at 9 Bar G It also has a fairly high peak at 10 7 Bar G which could be considered unacceptable This leads onto the next part Optimising the Controller We must now consider the effect of varying the gain As we wish to increase the rate at which the solution stabilises at our desired value the gain must be increased The three graphs below show the results when the gain is increased to 0 1 Bar G by increments of 0 025 Bar PIPENET
21. is what we should first attempt to change as it affects both the proportional and integral terms as well as any differential term we may have had As such it has been reduced to 0 025 Bar PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Gain 0 025 eee Peak has increased but is worth it to remove instability i Convergence still fast no 9 97 oscillations E Outlet pressure of valve 1 N Setting of valve E Outlet pressure of valve 1 low As can be seen the instability has been removed by the decrease in gain For this we have lost a little of the convergence speed and our pressure peak has increased It would be up to the engineer to decide whether this was acceptable or not and whether the sort of pressure increase seen here was likely The Effect of the Network lt is very important to note that one control system does not fit all and that they need to be tailored to the network in which they are installed To demonstrate this we will make a few small changes to the network Firstly we will demonstrate the effect of increasing the pipe length to 300m from 200m using our previously optimised network PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 300m Pipes 9 805 0 72 9 704 ja 70 64 9 604 0 60 9 504 70 56 y 9 405 _ 5 2 10 523 a 3 The system becomes 9 304 unstable 0 48 9 204 9 104 AAAI 9 00 8 900 0 E Outlet pressure of valve 1 E Setting of valve E O
22. iven example the pressure change occurs in a space of 5 s and any pressure waves present can travel throughout the entire network in fractions of a second PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 ca meaning that an initial guess for the reset time can be made of the order of a few seconds lf an appropriate reset time is not found it is best to simply consider a 5 second reset time and then reduce it incrementally until satisfactory results are achieved Due to the presence of integral control Anti Windup must also be considered This is used to improve the response time of the controller in the event of overshoot For example were the valve fully open but the pressure not yet high enough the controller would continue to increase its output value even though the physicality of the situation would not allow it Were the inlet pressure then increased and the control pressure then higher than required the controller would take a long time to respond as its output value would be greater than the one it should be at Anti Windup prevents this and as such will be used in this scenario Tracking time is the constant used in the anti windup equation used to determine how quickly the controller will prevent overshoot but it also must be calibrated However due to the care required in calibration most integral based controllers have a default value that is unchangeable PIPENET also comes with this facility and its use is recommended above s
23. know it can be of the order of seconds we can reduce the reset time to something lower say 1s Reset Time 1s 9 90 70 92 Much lower peak 10 88 9 804 Improved stability smoother curve 70 84 9 707 0 80 9 50 0 76 9 507 0 72 o 5 z 2 S 2940 0 68 3 a Ei rf 70 64 9 304 70 60 9 205 Much faster convergence 0 56 9 10 Good stability 0 52 0 48 8 907 T T T T 1 1 1 T T 1 1 T T 1 1 1 T T T r0 44 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Time seconds E Outlet pressure of valve 1 E Setting of valve E Outlet pressure of valve 1 low This initial reduction has shown a marked improvement in the convergence and stability of the system Looking now at what happens if we reduce further Reset Time 0 2s Convergence is faster 9 36 70 92 P Peak has d as 9324 ressure Peak has lowere 0 84 9 28 L 10 80 9 245 0 76 9 205 0 72 99 16 0 68 3 3 Pe A 3 3 1 Very unstable 4 f 3 9124 0 64 5 9 044 9 08 Mi 9 00 1 LL 8 964 8 927 Ba Tre OCE EOP ee a ee ET TC E a cc a ee ee ee ee CR RI ee LR ee 1040 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Time seconds PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 By reducing the reset time further we have once more introduced instability into our network which is undesirable despite the improvement in pressure peak reduction and
24. o achieve a smooth changeover to a standby pump Network The network below represents a typical pressure control system The PID controller regulates the valve s position to maintain a constant pressure of 9 0 Bar G at the valve s outlet The simulation will model the response of the control system when the upstream pressure changes from 15 Bar G to 20 Bar G pp PID Controller o LF9 Transfer Function quan Pressure Sensor 15 20 barg 5 barg Achieve 9 Bar G pressure PIPENET NEWS Model Settings VOLUME 2 ISSUE 11 SPRING 2013 The only changes from the default settings are to change the simulation length to 100 s and set a user defined timestep of 0 02 s Network Options scie zii Simulation times Simulation starts sec simulation stops sec User Defined Timestep Calculation timestep sec Ambient conditions Ambient pressure Bar A Ambient temperature 20 Force options Output total forces Output dynamic forces Time to start Dynamic Force Results sec Pressure Drop Model Coulson and Richardson Colbrook White 100 Variable Timestep 0 02 Defaults Roughness mm Hevation m K4actor Diameter mm Wavespeed Default wavespeed m sec 1260 E User Defined Pipe Wavespeeds Cavitation No cavitation Vapour cavitation only 2 Channel cavitation only Dry Pipe Treat all pipes as wet New pipes wet New pipes dry Cancel Apply Fo
25. of values In simple networks such as the one shown a simple technique can be used to gain a good first approximation It is simply a matter of running the network with no control loop at all The aim is to see how the system would respond to the valve slamming shut at the maximum possible pressure thus generating the greatest oscillation Using the following network with the associated valve closure profile the following graph is obtained c 08 Informatio o ee eo gt 0 oO 0 10 20 30 40 50 ee 20 barg 5 barg PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Graphical Results From Simplified Network ik AnA hha eee 3 III WW VY Vy i II II Vy II AN AA yy gig VI N The graphical results show that the maximum oscillation caused by this scenario is approximately 18 4 Bar G This means that the initial gain should be set to counter this at l 1 Gain 194 Bar 0 054 Bar While this is a simple procedure for such a basic network in larger networks it can quite clearly be seen that this could become impractical In these situations the first approximation for the gain should be left to engineering experience In case of doubt it is always b
26. r calculation options we will look at setting the initial state This is normally not Calculation options Controls Absolute tolerances Relative tolerances Initial state Output 3 fmm Initial starting point 9 Default Static all flows zero Read from initial guess file Initial guess file None Bro Runin time sec 200 Default V Calculate initial steady state E Calculate final steady state recommended for use in networks with control systems as they can sometimes diverge in the initial state calculation However as the network is simple with a long enough run in time a steady state can be reached quite satisfactorily meaning we can benefit from Starting the calculation from a steady state solution Note the long run in time of 200s In addition to these the units selected should be Metric the fluid should be Water at 20 C which is the default setting and the pipe schedule selected for use in pipe types should be ANSI 36 10 Schedule 40 PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 Network Data The following data should be entered for the components in the network Both pipes are the same with the properties window below denoting the required inputs Copy Paste can be used to achieve this if set on one pipe Label Input node Output node Type AnsiB3610 v Diameter 100 El mm Length 200 i Net height change 0 m Roughness 0 0457 mm Additional k factor 0 W
27. the effect of events and the network must also be considered A simple network is studied here to highlight their interaction but the principles outlined below are applicable to considerably more complex systems In order to understand these principles we must first recognise that the way controllers are tuned in the real plant and the way they are tuned in PIPENET is the same In other words PIPENET simulates this process whereby plant controllers are tuned during commissioning Which begs the question why would it be necessary to model the control systems in PIPENET in the first place PIPENET can help to determine whether the control system s manner of operation is inherently sound For example the level control of a tank can be found either on the outlet or the inlet or can even use a simple switch rather than a PID controller If it is a switch it must be determined whether any hysteresis is required In modelling these systems with PIPENET it is considerably easier and cheaper to determine which options are best Suited to the application In the event of instability arising from the control system PIPENET can be used to find the cause of the instability and troubleshoot well before any construction has begun It can also be used to assess the benefits of using a cascade control system as well as being used to determine the control philosophy For example in the event of the run down of an operating pump what would be the best way t
28. utlet pressure of valve 1 low This can be fixed by reducing the gain once more although here only to 0 05 Bar 300m Pipes Gain 0 05 70 72 10 07 0 68 Pressure Peak increased 9 94 0 64 9 8 0 60 9 74 1 0 56 9 65 3 a 5 amp ___S 9 57 0 523 9 47 048 931 Oscillations removed 0 44 9 27 0 40 9 14 Ne 7 1 0 36 90 Le Le ae 8 94 rT di Toe T T el DE E a ae DE A POT PT r a ae ae T T i COLO F Ys os a ae a T TT 7 T E CC E E 0 32 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 E Outlet pressure of valve 1 E Setting of valve E Outlet pressure of valve 1 low Effect of Constituent Components in the Control System lt must also be noted that the same control system will also not work for a different valve in the same network Returning to our original network we will now see what happens in the event of a different valve closure profile Currently we have assumed a linear profile which PIPENET NEWS VOLUME 2 ISSUE 11 SPRING 2013 means that the valve closes at a constant rate Here we will look at the difference between a linear profile and a first order profile A first order profile is one which will approach the final steady state position quickly and slow down as it nears that position To demonstrate the differences in the valve closure profiles we have chosen the control parameters which generate instability namely a gain of 0 1 Bar and a reset time of
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