need ANSI600 outlet flange on pilot PSV but only ANSI150 available 1

[zyllaron]

The suction pressure of the triplex pump can be as high as 845 psig.
The PSV set at 1440 psig on the discharge relieves to the suction.
Because of the potential of high backpressure we believe a pilot PSV is required.
However, the vendors only have ANSI-150 outlet flanges for pilots with ANSI-600 inlet flanges.
One vendor refers to API 526 table 17 that we will not be able to find ANSI-600 outlet flanges on a pilot with ANSI-600 inlet.
Does anyone have advice to accommodate the high backpressure on this PSV?

 

[bljnv]

I suggest you can review the application. PSV are meant to relieve into atmosphere or other vent lines. Venting the PSV into a pump suction line is a thing which I have not come across. What is this application?
PSV for class 600 inlet valve will be class 150. For class 900 and above 300 class flanges are available. Other class flanges can be provided as a special case. But this will require a modification of the valve casting and can be provided only if there are no other dimensional conflicts. Also please note that in this case the valve body chest portion will be designed to base class ie. 150 or 300, and only the flange is modified. So the modified valve with different class flange can take only the original pressures it is designed for though it can be physically fitted on to a higher class flange.
Also with high back pressure the valve performance will be affected. Valve can chatter, and its capacity will be affected.
Regards.

 

[LittleInch]

You will probably need a pilot operated control valve for this purpose as the duty is quite strange for a simple PSV. Many people make simple pilot / spring operated control valves which open / relieve at a set pressure and require no outside power.

My motto: Learn something new every day

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

 

[zyllaron]

bljnv - The application is: fluid: natural gas hydrocarbon condensate liquid (condy)from an inlet separator; pumped to atmospheric storage where the condy is weathered to flare. On blocked flow the discharge PSV (set at 1440 psig) is to relieve back to the inlet separator that has a PSV set at 825 psig.

Question to All - Here's a fundamental question: How can a pilot PSV be used for one of it's main intended purposes (i.e. back pressure greater than 40% of set pressure) if the discharge flange rating only allows up to 20% of set pressure (285 psig for ANSI-150 divided by 1440 psig)? We thought we could specify a pilot PSV with ANSI-600 inlet and ANSI-600 outlet because the back pressure is greater than that for ANSI-300 (740 psig). But we have not been able to find a pilot PSV that allows it to function any better than would a balanced PSV. A PSV manufacturer tells us they can custom make one for us but it will not be ASME UV certified. I get the feeling I'm missing something obvious.

 

[bljnv]

Pilot operated PSV are meant to hold the system pressure up to 98 percent of set pressure without any leakage. A small pilot valve is provided in a passage of main valve. The passage connects the main valve inlet and spring chamber side ie. over the disc. The closing force of the main valve is spring force plus the system pressure force. As the pressure force of inlet lading fluid is balanced by the same amount of pressure force acting on the top of the disc. in fact the area over the disc is comparitively larger than inlet area. Hence the valve is held leak tight upto 98 %. The pilot valve is normally set to relieve at 2 percent lower than main valve setting. Once the pilot valve opens at 98 percent of main valve setting the load over the main valve disc is only due to compression of spring. After the pilot valve pop point main PSV is activated. Pilot PSV will not be suitable for this application
As I understood your application looping the PSV discharge back to a pressure source is not correct. Please review.

 

[Duwe6]

This sounds more like a 'process' related valve. Thus the 'UV' stamp is not really needed. As bljnv states, this does not appear to constitute a correct PRV installation.

Other than blocked flow, what are your relieving scenarios? Did you include Fire?

 

[moltenmetal]

Stop trying to use a PSV as a back-pressure regulator. Buy a back-pressure regulator instead.

 

[zyllaron]

bljnv - pump overpressure protection relief back to the source vessel is common practice. You give a description of one of the advantages of pilot PSVs. Another major advantage of pilot PSVs is that they can tolerate more than the 10% backpressure that conventional PSVs are limited to, and more than the 40% that balanced PSVs are limited to. You then state, "Pilot PSV will not be suitable for this application" but it appears to me you provide no rationale for that statement. Please explain.

Duwe6 - the governing case in blocked flow. The thermal expansion and fire cases have much lower required flow-rates.

Moltenmetal, does a back-pressure regulator on a PD pump satisfy the ASME VIII and API 521 requirements for last line of defence for overpressure, for example from blocked flow? I've seen several clients and EPCM put PSV protection on PD pumps that already have manufacturers' internal back-pressure regulation. Are you saying the PSV is not required if a back pressure relief valve is in place?

 

[don1980]

zyllaron - what is this PSV protecting? That answer is needed in order to understand the allowable protection options. For example, if the PSV is protecting just the pump, or the pump and piping, then you can choose design options other than relief devices. You can use a bypass valve or minimum-flow recirculation valve (or orifice) rather than a relief valve. On the other hand, if you're protecting a pressure vessel, then a relief device is needed. I'm guess that this PSV is installed to protect the pump and piping, which would allow options other than relief devices.

One can typically route the relief device back to the suction side without experiencing such high pressure. The fact that this isn't true for your application means that you need to consider routing the discharge elsewhere, to another vessel, if indeed you choose to use a relief valve.

 

[bljnv]

Conventional PSV can very well used for back pressures higher than 10% of set pressure. But one need to consider back pressure correction while arriving at the capacity. Balanced bellows valve does not get affected by back pressure. PSV is a device expected to pop open at given pressure, attain full lift and relieve full flow within the specified over pressure. Upon connecting the discharge back to the pressure source
, you are not allowing the PSV to reseat. I agree with don1980. If you are intent is only to protect pump, PSV is not required. You are the better judge as you know the condition better. All the best!

 

[moltenmetal]

A star to don1980 for clarifying things.

 

[zyllaron]

don1980 The governing purpose of the PSV is to protect the pump and piping from overpressure due to blocked flow. I need to look this up but my understanding is that inside the plant, ABSA (Alberta Boiler Safety Authority) requires adherence to B31.3 which I believe specifies that all pressure piping and equipment be protected according to ASME VIII which specifies that protection must be by a PSV that is certified by the National Board (NB-18 what we used to call the Red Book). Do you or others following this thread have a reference that shows a proportional pressure relieving valve satisfies the requirements of ASME VIII for this pump and associated piping? And/or have I misunderstood what is the required relief device for protected systems.

 

[LittleInch]

I think you've misunderstood the relief system.

What I think the system should have is a re-circulating control valve / ARV set below your design pressure as your primary operational control to prevent over pressure in the event of locked in flow. Then the PSV becomes a more straightforward PSV which I set at your piping design pressure in the event that the re-circulation valve fails to open for blocked flow. where you send this potential flow to is a different matter as it can't go back to the separator, but this instance I would suspect it would need to be an ESD trip of the pump (2oo3 pressure switches)and probably the whole system.


As don 1980 says look at the system you're protecting and what it's design limits are and what the max pressure the pump can put out before it stalls. Piping has some fairly high potential over pressure allowances for short duration (30% from memory for < 10 hrs a year).

I think you need to have a discussion about your protection philosophy and how the plant protects itself (ESD trips etc) and where PSV discharges go to.



My motto: Learn something new every day

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

 

[don1980]

zyllaron - you'll notice that only a small percentage of piping segments have a relief device, while almost all pressure vessels have one. That's because the pressure protection rules (ASME B31.3 and ASME Sec VIII) are different. The core difference is this: relief devices are required, by default, for pressure vessel, while a relief devices are required by exception for process piping. That's why I asked, in an earlier post, what you're protecting. That answer defines the options available to you.

ASME B31.3 doesn't say that all piping has to be protected according to Sec VIII. If it did, you'd see a relief device on all segments of piping. Instead, B31.3 says that the piping must be designed to either safely contain or relieve the pressure to which it may be subjected. So, for piping the owner has a broad range of discretion. In the application you describe, you have a range of design options for preventing overpressure. For PD pumps, a relief valve is commonly used, but in this specific application a relief valve doesn't look like a realistic option. What I am saying (and others too) is that you don't have to use a relief valve - you have other options.