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3

# Pressure Regulators Vs Control Valves

10 replies to this topic
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### #1 panagiotis

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Posted 19 June 2024 - 12:54 PM

For pressure regulators there is not the definition of Kv or Cv value.

Normally, when we know the Cv value of a control valve for 100% opening, then we can calculate the maximum flowrate for the control valve.

For a pressure regulator, Cv or Kv value is not defined or given by the manyfacturer. Then, how can we calculate the maximum flowrate in case that fails open?

### #2 fallah

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Posted 19 June 2024 - 01:08 PM

For pressure regulators there is not the definition of Kv or Cv value.

Normally, when we know the Cv value of a control valve for 100% opening, then we can calculate the maximum flowrate for the control valve.

For a pressure regulator, Cv or Kv value is not defined or given by the manyfacturer. Then, how can we calculate the maximum flowrate in case that fails open?

Hi,

The Cv can be defined by the manufacturer of PCV in the relevant data sheet if s(he) is being requested to define, but a flow curve which is provided by manufacturer is the best way to determine a regulator's performance for specific application instead of Cv value.

### #3 panagiotis

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Posted 19 June 2024 - 01:39 PM

When we say the set pressure of the pressure regulator is 5 barg, we mean that the outlet pressure is 5 barg, right?

Also, the secondary pressure of the pressure regulator is the same as the set pressure?

Let me ask you something more specific:

If I have a pressure regulator and downstream I have a PSV with a set pressure at 6 barg.

Then if I the overpressure scenario is fail open of pressure regulator.

Then from the flow curve (I am attaching a random example) I can read what is the maximum flowrate through the pressure regulator and this will be the relief load. Right?

Normally these curves they give one inlet pressure (let's say 10 barg) and different outlet or secondary pressures.

These curves are constant, but at the end there is a reduction.

In the example below, If we assume that the set pressure is 6 barg, then as maximum flowrate I would take around 22000 l/min (red arrow)?

### #4 fallah

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Posted 19 June 2024 - 05:10 PM

When we say the set pressure of the pressure regulator is 5 barg, we mean that the outlet pressure is 5 barg, right?

Also, the secondary pressure of the pressure regulator is the same as the set pressure?

Let me ask you something more specific:

If I have a pressure regulator and downstream I have a PSV with a set pressure at 6 barg.

Then if I the overpressure scenario is fail open of pressure regulator.

Then from the flow curve (I am attaching a random example) I can read what is the maximum flowrate through the pressure regulator and this will be the relief load. Right?

Normally these curves they give one inlet pressure (let's say 10 barg) and different outlet or secondary pressures.

These curves are constant, but at the end there is a reduction.

In the example below, If we assume that the set pressure is 6 barg, then as maximum flowrate I would take around 22000 l/min (red arrow)?

An inflection point where the flow curve starts to drop off is point of choked flow in which increasing the pressure drop through a PCV no longer results in an increase in flow.

In the example you provided, the inlet pressure is 10 bar, the outlet pressure set at 6 bar (for upper curve) and the maximum flow (choked flow) passing through the PCV while is in wide open position is around 22,500 l/min.

### #5 breizh

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Posted 19 June 2024 - 10:50 PM

Hi,

On top of Naser's reply , consider this link with example.

https://www.controla...sure-regulator/

Breizh

### #6 shvet1

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Posted 19 June 2024 - 11:54 PM

Then from the flow curve I can read

Avoid confusuing curves, use ISA-75.01.01. There are many calculators available in google, free or paid what one will prefer.

All you need to calculate PSV capcity is to find Cv @ stem full opening. If the CV has not bought yet we practice specifying max Cv allowed in CV datasheet,

Manufacturer can limit max Cv by stem limiter.

Edited by shvet1, 20 June 2024 - 04:49 AM.

### #7 panagiotis

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Posted 20 June 2024 - 04:40 AM

Hi Fallah,

Thanks for your answer. I understand the way you interpreted the graph.

If downstream of this regulator there is a PSV with a set pressure for example 6 barg. Then, if I want to estimate the relief load when the pressure regulator fails open, I cannot use the above graph to estimate this flowrate, right? Normally fail open scenario is for 100% opening of a control valve.

### #8 fallah

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Posted 20 June 2024 - 07:34 AM

Hi Fallah,

Thanks for your answer. I understand the way you interpreted the graph.

If downstream of this regulator there is a PSV with a set pressure for example 6 barg. Then, if I want to estimate the relief load when the pressure regulator fails open, I cannot use the above graph to estimate this flowrate, right? Normally fail open scenario is for 100% opening of a control valve.

Hi,

If the PCV will be failed in wide open position (failed open scenario not blocked outlet), the downstream pressure is mostly going to be higher rather than to be subject to pressure reduction, then the graph info cannot be applied. on the other hand, if the PCV set point is 6 barg then the PSV set point cannot be 6barg and should be higher.

At first you should specify the scenario at which the PSV will be open then considering the conditions of downstream facilities, the pressure at PCV downstream should be estimated at PCV fail open condition. If design pressure of the downstream facilities is 10 barg or higher no need to PSV for overpressure scenario otherwise the PSV should be set at design pressure of downstream facilities.

Edited by fallah, 20 June 2024 - 07:36 AM.

### #9 panagiotis

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Posted 20 June 2024 - 11:29 AM

Hi Fallah,

Sorry I put in my question the PSV and then there was confusion between the set pressure of the PCV and PSV.

Let me ask differently if I want to calculate the maximum flowrate across the pressure regulator for fail open. How can I approach it?

For a normal control valve we know the Cv value (for 100% open) and then by knowing the upstream and downstream pressure we calculate the flowrate, which will be the max flowrate.

For a pressure regulator, where we do not have the Cv value, but only the graph that I sent you. How could I calculate the max flow through it in case it fails open.

### #10 fallah

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Posted 20 June 2024 - 12:49 PM

Sorry I put in my question the PSV and then there was confusion between the set pressure of the PCV and PSV.

Let me ask differently if I want to calculate the maximum flowrate across the pressure regulator for fail open. How can I approach it?

For a normal control valve we know the Cv value (for 100% open) and then by knowing the upstream and downstream pressure we calculate the flowrate, which will be the max flowrate.

For a pressure regulator, where we do not have the Cv value, but only the graph that I sent you. How could I calculate the max flow through it in case it fails open.

Hi,

Please pay accurate attention to the follows.

There are two different situations for a PCV based on which it can be in wide open position:

1-The pressure at downstream goes down at a high rate hence the PCV have to be wider open to compensate the pressure shortage at downstream till a limit as choke point (corresponding to fully open position) at which the flow is in maximum value and further pressure reduction at downstream doesn't increase the flow passing the PCV. In fact, the Cv of a PCV is normally used to define the point of the choked flow, a mean of Cv totally different than that of the control valve.

2-The internal mechanical failure of the PCV leads to having momentarily fully open position at which the downstream of PCV is mostly going to be pressurized higher than (or at least equal to) the set point with a passing flow certainly lower than the choked flow due to lower pressure drop than that value has been in the first situation.

Therefore, you can see the concept of Cv in PCV is different than that of the control valve and the choked flow in the curves/graphs provided for PCV indicates the maximum flow passing through the PCV in fully open position.

### #11 panagiotis

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Posted 20 June 2024 - 02:37 PM

Thanks Fallah for the detailed answer!