propsignclosure rule for FCV

Question

Dear Scott: 
 Kindly, as per your reply: "However, the relative closure for this case is 89.5%. You state in your email that you want a relative closure of 5%, however this will not correspond to the flow that you want. If you need the flow to match the flow from the FCV, you will need to use the larger relative closure for the start of the operating rule". This is was my mistake. Now I realized that I need to keep the values of initial Cv = 0.04166 m^3/s/(m H2O)^0.5 and full open Cv = 3.47878 m^3/s/(m H2O)^0.5, then I add an proposed operating rule as shown where the starting relative closure is 89.501%. As result, the maximum surge pressure 324.048m at J-63, and negative pressure at the downstream -9.984m. from this operating rule: is this mean that at 0.0 sec the % of closure is 89.5% and after 10 second it opens where the relative closure = 0%, then, after 20 second its closes to 60 % and to 100% after 30 sec.?

If I compared with other operating rule keeping starting relative closure is 89.501%, as follows: Time from Start (sec) Relative Closure (%) 10 0 20 80 30 100 50 100 
 The results shows that the max. Pressure from flex tables= 324.062m, 
 Other closure rule, Time from Start (sec) Relative Closure (%) 10 0 20 100 30 100 50 100 
 The results shows that the max. Pressure from flex tables= 324.062m, the effect of changing the closure is low, but however, why it give lower pressure than the previous one although it closes 100% after 20 seconds? Other closure rule, Time from Start (sec) Relative Closure (%) 10 0 60 50 80 100 100 100 
 the result shows that Pressure from flex tables= 330.771m, the question is why it gives higher pressure value although at 60 sec the closure is 50% if compared with previous fast closure time.? 
 What is the good closure rule to keep the system safe or proposing sudden closure? 
 Other question, I tried to use as a trial; the initial Cv =0.06 m^3/sec/(mH2O)^0.5, which gives calculated relative closure = 88.1%, if I created closure rule using the same first closure rule 
 Analysis without PRV upstream the FCV.zip The results give max. Pressure = 325.826, is this trial acceptable, since its keep the TCV working as FCV without causing problems as the first email i sent.

Scott Kampa · Answer

Hello Sawsan, 
 
For your patterns, are you trying to have the valves start out close, then open fully, then close again? That is how the patterns are currently set up. I have a starting relative closure of about 89%, then the pattern has a 0% relative closure after 10 seconds, and the closes over time. If you look at a Time History graph for the flow at the TCV with your current setup, you will see the flow increase early in the run (because the valve is becoming fully open) and then decreasing again as the valve closes. 
 
However, if the valve is simply supposed to closer over time from it's starting point of about 89% closure, you will need to adjust the pattern to remove points less than the starting relative closure. As an example, I changed your pattern to the following: 
 
Time from start Relative Closure 
10.0	89.51 
20.0	95.00 
30.0	100.00 
50.0	100.00 
 
This would be a more realistic pattern for a valve starting at 89% relative closure and then becoming fully closed after 30 seconds. (This will not necessarily reflect your actual system. You should set the pattern to accurately reflect the expected closure for the valve.) The closure with this type of pattern will be slower and the transient that occurs will be less severe. 
 
Regards, 
Scott