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HOW TO DETERMIN THE SIZE OF RELIEF CONTROL VALVE FOR MAIN

findfpso (Chemical)
13 Apr 05 4:30
I am request to calculation the size of relief control valve and the background is as following:

   three(3) centrifugal pumps use one ring of DN400 pipe, the customer want to determine the size of relief control valve if three or two pump run in same time, in order to protect the system not reach high pressure to damage the pump and system.

   Capacity of pump:  1200m3/h@13bar
   Control valve setting pressure: 15bar.
   flow fluid: seawater @20 deg.

so anyone can give me some advice to determine the size of control valve?
   
Thanks!
Montemayor (Chemical)
13 Apr 05 11:20
findfpso:

One reason for favoring a centrifugal pump in a process application is that, if the piping is designed and installed appropriately, there is NO PSV required to protect the piping and associated equipment due to the pressure increased by the pump(s).

In other words, the maximum allowable working pressure (MAWP) of the piping and the pumps is selected as a value beyond the maximum, reachable pressure of the centrifugal pump(s).  This is considered perfectly safe and credible - always taking precaution that the [b[correct[/b] impellers & speed are always applied to the pump(s).

If you have a situation where the MAWP can be exceeded by the pump(s) discharge, then you must protect the system by taking into consideration the LOWEST MAWP in the system.  In other words, attached equipment may have a lower MAWP than the circulation header (often the case).

You write about a "control" valve.  In a pressure safety relief scenario, a control valve is not applied.  The instrument recommended is a PSV, a pressure safety valve - which is self-contained, and does not normally depend on other instruments to aid it in supplying the pressure relief capacity.  Is there a possibility that you mean to control the capacity of the pump(s), and not the discharge pressure?  That would not necessarily constitute a safety scenario.

MJCronin (Mechanical)
13 Apr 05 15:05
findfpsoa and Montemayer....

I believe that you have to address the case where the minimum flow rate of "high energy" (multistage) pumps is not met. (for example, in the case of multipe boiler feedwater pumps feeding a common header that feeds multiple steam boilers)

Designing a protection system for pumps based on only pressure may not be adequate. Multiple pumps running might generate excess heat and bearing loads at operating conditions far below the MAWP of the weakest component...

For this type of situation, the control valve setpoint would depend on how many pumps are running and the sum of the minimum flow rate for each running pump.

High energy pumps can fail when operated below the min flow point to which they were designed

My opinion only....

MJC


MJC
Basing a

  

Montemayor (Chemical)
13 Apr 05 16:42

I have only addressed the supposed safely requirement of  protecting the pump(s) and associated piping against an overpressure situation.  The capacity involved in such a scenario is what determines the physical size of the required PSV, and would be the sum of the capacity contributions of the pumps working in parallel at the cumulative pumping condition - which may be different for any one pump.

I have not considered controlling the capacity of the pump(s) nor protecting them from minimum flow or other conditions specific to their design or operation.   I leave that for another query or further considerations since we don't have any basic data on the pumps characteristics - other than that they are centrifugal type.  They could be multi-stage, turbine-regenerative, etc.

Again, perhaps the question is redundant, but what purpose does the expected "control" valve serve?  Is it for over-pressure relief or is it for volumetric capacity relief?

findfpso (Chemical)
13 Apr 05 20:27
hi,montemayor:
  Thanks for your careing. in fact, it is three fire pumps, NO.1 & NO.2 + one emergency fire pump, the working status is as following:
  
    1. The system is a wet system, if fire, the pressure inside pipe will down, but if down to 7bar, then NO.1 pump started, if the pressure still down to 6 bar, then NO.2 pump start, but if the pressure is still down, then emergency pump start, thus three pumps run in same time, if there no enough capacity to consume, then the pump will run in a closed pumping system, will cause  pump heat and then damage the motor.
      That's why to request install a pressure self-control
valve, when the pressure exceeds 15bar, the valve will open tto relief the capacity to keep the pump run safely.
     
     I do hope someone to give me some method to calculate the size of this valve and brach. also if will be appreciate if someone give me the method to calculate the size of PSV.

 Many Thanks!

 David Jang
TD2K (Chemical)
14 Apr 05 2:17
I'm a little confused what you are trying to do here.  A PSV is typically installed to protect equipment or piping from overpressure.  At 15 bar pressure, have you reached the MAWP of the firewater system or the pump or other equipment?

If you are trying to size a minimum flow valve for the firewater system (as you seem to imply by "the valve will open to relief the capacity to keep the pump run safely" given your previous comment about not having enough flow through the pump and it could be damaged, that's a different approach.  You also want a control valve, not a PSV IMO.

To size the control valve for the minimum flow to protect your pumps I would look at your pump curve(s).  Either the curves, or the data sheet or failing that, the manufacturer will be able to advise you what the minimum flow is.  Then, do you want to size the valve assuming 1, 2 or 3 pumps are running?  If the curves are identical, sizing the valve is then trival, it's your minimum flow times the number of pumps you decide you want to assume could be running.  The inlet pressure for the control valve is the head produced by the pump(s) at that minimum flow.

If the pumps have different curves, it then gets more difficult to come up with a flow that protects all 3 pumps.  This is because a pump will a lower head curve than the others can be totally 'backed' out of the system as it can't produce the necessary pressure to get into the main header (that will commonly be the case with a jockey pump).  If that is the case (eg. you have different pump curves), you'll need to look at the curves and identify a maximum water pressure before the 'worse' pump approaches being shut-out.  For that pressure, you read off the flow at that pressure from the curves for the pumps you assume could be running and add the total flow up.
quark (Mechanical)
15 Apr 05 2:43
Couple of questions.

1. If there is no enough capacity to consume, how will the pressure drop?

2. How can you be sure that the pumps will not run continuously when the relief valve is installed.

The simplest way is to install three good pressure switches and set them with 1 bar pressure difference. This is what you have right now. When there is no enough capacity to consume, pumps will not be started.

This is a standard arrangement for fire hydrant systems. Please note that you should check your local codes about automatic pump operation. We practise not to stop the main hydrant pump automatically. We manually stop it when the fire is extinguished. However you can interlock the jockey pumps with main pump in such a way that jockey pump is off when main pump starts.

Regards,

findfpso (Chemical)
15 Apr 05 3:42
hi,quark:
  
   1. the purpos of seting a relief valve is for protect the pump during the time between fire stoped and manual stop the pump. you are right, if the fire pump started, it can not remote stop.
   2. because it can not stopped by remote control , so we have to install a relief valve during three pumps running.

  it is nothing to do jockey pump, because the capacity pump is very small and only 50m3/h.
 
  The purpose of relief valve is to keep normal pressure in system.

thanks.

David Jang

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