Valve Coefficient for a Back Pressure Valve

[Berenger]

Hi everyone,

I am trying to calculate the CV for a back pressure valve (BPV). I have a BPV holding the pressure in a vessel to 50 psig, i.e the BPV is set at 50 psig. How do I calculate the CV for the valve? CV is dependent on the inlet pressure and pressure drop. However, since we have a BPV, the inlet pressure to the valve will always be 50 psig unless the valve fails. Even if the pressure in the vessel builds up, the BPV adjusts it's orifice opening to get the pressure back down to 50 pounds. Hence, the inlet and outlet pressure ideally should be around 50 pounds (give or take a few).

So, what pressure and pressure drop values do I use to calculate the CV? If BPV works as it is supposed to, then pressure drop should be 0.

Also, what will be the maximum inlet pressure possible? If I understand correctly, the inlet pressure will always be around 50 pounds (unless there is a failure).

I will really appreciate any assistance that I can get from our more experienced colleagues on this site.

Thank you all.

Berenger.

 

[LittleInch]

What is it with everyone trying to be a valve engineer? Just put down your duty requirments and send them to some control valve suppliers who will tell you what their valves can do.

correct, your inlet pressure remains(hopefully) constant. Howwever your flow and downstream pressure presumably don't otherwise there is no where for the excess pressure to go. All a control valve can do is open and close. It can't increase pressure on it's own. Hence what you need to find is the range of downstream presusres and flows.

To calcualte the range of CVs for your valve you need P1 (your 50 psig), your options for P2 - generally take the min and max, and then the flows at those points. The trick is to try and find the min and max flows and min and max DPs and see if they occur at the same time.

If you need it to seal at no flow then say so as not all control valves do this.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Berenger]

Thanks, LittleInch, for the help. By the way, i'd rather send this to a valve engineer. But, for some reason, the senior engineer wants it done in-house. We've always sent control valves and regulators out to be sized.

 

[LittleInch]

No problem,

This is a control valve though, just with its pressure control point taken from upstream. Just because some people have this strange tendency to add words like flow, pressure and temperature in front of the word control valve doesn't matter to the valve - it's just a control valve controlled by different inputs. I've had control valves with 5 different inputs before now running through a low select filter so that the valve takes the position of the lowest opening. Your valve will have a CV even when fully open so there will always be a pressure drop, but most designers work on an opeing span of around 20 to 70 to 75% for normal min / max conditions.

You can always ring one or two vendors up instead of e-mailing them....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Berenger]

LittleInch, I think I have quite a good grasp on regulator operations and design equations. I am still a little confused on what the pressure drop will be in some situations (such as when the regulator is a last layer of protection and is not expected to be activated). I have included a scenario that may help clarify my question.

If you look at 12321, you'll see that we have a control valve set at 47.5 psig to control the pressure in the vessel. Hence, the outlet stream pressure (depicted by the bold line) will always fall very close to this set pressure. Now, I am clear on this so far.

On that same outlet line, we have a secondary abatement (the BPV) with a set pressure of 62.5 psig. The way I understand it, if the PV fails, the BPV kicks in. The line from the BPV goes to the flare knock out drum with operating pressure of 50 psig.

Now my question is "in order to size the BPV, what will be the downstream (outlet) pressure from the BPV? Hence, what will be the pressure drop across the valve?" All simulations I have done show a worst case scenario of 48.5 psig. Can I size the valve for 0.1 psig pressure drop? 1 psig? The maximum inlet pressure into the vessel is 49 psig, so realistically, the pressure can't exceed that unless there is a cause (such as a misaligned valve) upstream. All scenarios show that the 50 psig operating pressure of the flare KO drum will be higher that the line pressure.

 

[Berenger]

Sorry, forgot the file.

 

[LittleInch]

OK, Sorry if I insulted your intelligence.

From what you've provided and stated I think your issue is contained in the phrase "The way I understand it, if the PV fails, the BPV kicks in. ". Note that the failure mode of the PV is actually Fail Open, but lets suppose someone closed the isolating valve or something happens furehter downstream.

It doesn't look like that to me as you have two different set pressures - 47.5 psig on your system controlled valve (as an aside I don't like manual bypasses around control valves - if you need a control valve you need a control valve, not some hairy ar8ed operator pretending to be one instead, but not everyone agrees with me...) and 62.5 psig on your BPV. Therefore the BPV won't open until it gets to 62.5 psig. If it has 50 psig on the downstream side going into the flare KO pot then it's pressure drop will be 12.5 psig or maybe a little less once you get some flow in the pipe.

If the normal maximum inlet into the vessel is 49 psig, then the BPV will never open, even if the pressure gets to 60 psig. It will only open if something causes the pressure in the vessel to rise above 62.5 barg which I guess is why the designers placed it there. I can only go on the snapshot of the P & ID you've sent and your description but you do normally need at least a couple of psi accross a valve like that for it to work effectively. You also normally need about 5% difference between set-points for each valve to work properly if there is a rising pressure at whcih each is supposed to open and control flow.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Berenger]

Thanks, LittleItch, for your help. No, you did not insult me...I was just trying to explain the extent of my understanding of the issue. That's what the forum is about - helping others, and you did just that. It's quite clear to me now. I really appreciate your assistance.

 

[AGomaa]

Berenger,
In addition to the valuable info that LittleInch provided, there are some fundamental issues that I would like to highlight:
- If I understand your drawing correctly, your control loop that controls the vessel pressure consists of (transmitter/Controller/valve), although the valve fail position is open, if you conduct HAZOP for this loop we can consider loop failure (regardless of loop components consider valve fail to open on demand) which can cause the pressure in vessel to increase to the limit that cause damage and or loss (loss of containment, Personal injury, pollution etc.). The safe guard for this would be by installing a Safety Relief Valve (SRV) and not a back pressure regulator. It is crucial to identify if this valve for safety protection or not.
- The BPV or the SRV shall be sized to allow all the gas that entered to the system to relief. So look at the upstream feed and assume that all vessel input gas will go through the BPV/SRV. In such case the inlet pressure will be 50 psig. The outlet pressure is your header pressure (max). This is general rule to protect the vessel from overpressure. To accurately design the system you need to show the vessel connections. Even for isolated vessels that heated up (by surroundings or fire) we install SRV just different calculations. No vendor will be able to provide the flow rate/inlet and outlet pressure, you have to provide all such data so they can size the valve.
- There is more in sizing SRV than the above but you can visit any of vendors sites to learn more about sizing SRV.
- One of the characteristics of (non-pilot) pressure regulators that it is designed around one set point so consider your regulator is maintaining the back pressure in the vessel at 50 psig and you passing quantity x of gases. If you want to pass quantity >x you need to open the regulator more which cannot be unless you increase the pressure on the regulator diaphragm meaning your vessel will be at > 50 psig. A pilot operated regulator will not have such problem.
- Even if you rely on the best vendor in market it is good idea to understand how they size the valves and you should be able to review their selection. At least you know the reason and limitation of their selection.

 

[LittleInch]

AGomaa,

Your explaination is quite good, but misses the point that the pressure that needs to be not exceeded is the design pressure of the vessel, not the header pressure. We don't know what this is, but have to assume it is at least 62.5 psig which is the set pressure of the backup BPV. If this BPV is acting like a safety relief valve then it needs to be of similar robust construction and operation and as you say, be sized for full inlet flow at the releiving pressure (in this case 62.5 psig).

The margin of the set point accuracy is a good one, but requres each valve not to overlap set points taking into account +/- accuracy. A non pilot valve will likley have accuaracy around +/- 10%, which is probably one reason why the set pressure from the BPV is set 15 psig higher than the normal control valve to avoid any overlapping opening, i.e. the BPV should not open before the normal CV is at 100% open.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way