Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here





Hi, first time poster.

I know there is a lot of discussion of PSV vs. PRV, which is not what I want to discuss.

I have the following situation at the plant I work at:

The client is re-certifying their PSVs. We have come across several PSVs (labeled as PCVs on the vendor P&ID) on the discharge of lube oil pumps for several compressors.

A previous engineer on the project changed the P&ID to read PCV instead of PSV, so I am bit confused...

Isn't there some guidance somewhere where a PSV is not supposed to be used as a PCV? Or am I mistaken? Some of these lube oil PSVs are tight shut off, as expected from a PSV. However I know of some others that are not... Maybe the lack of tight shut off is why it was labeled as a PCV?

The configuration is as follows:
Lube oil pump, circulates oil, PSV/PRV/PCV dumps it back into the reservoir if the pressure exceeds 100psi

So in this case is it a PSV/PRV or a PCV?

For the record I am Mechanical Engineer tasked with getting these valves out to their third party calibrator for repair and put back. Since it is coming off the discharge of a pump, I am of the opinion that IT IS a PSV/PRV, even though the P&ID says other wise...


Can you advise on what "valve" is actually installed ? Brand, type, size etc.? Should then be able to deduce whether it is a PRD or PCV probably by brand name alone. In any case, the system you describe should have a PRV in any case. Not sure why a CV would be there. Do you have a data sheet of the valve ?

Per ISO, only the term Safety Valve is used regardless of application or design.


Unfortunately I cannot find a datasheet in our records, which is unusual. The compressor vendor had a blurb in their notes saying the PSV was supplied by the lube oil pump vendor. I did track down some stuff specifying the valve make/model but no relief calcs or anything like that.

However, I am still digging around. Below is the information you requested.

Manufacturer: FulFlo
Inlet: 1-1/4" Class 300
Outlet: 1-1/4" Class 300

Assembly Number (Confirmed between paper and what is in the field)

Vendor catalog here:


S relief valve is a safety device. A control valve is a control device. In my experience a relief valve cannot throttle its position and a control valve must throttle its position. I do not ever include a throttle valve as part of an isolation scenario (i.e., after a valve has throttled flow for a while it becomes progressively less likely that it will seal off flow when fully shut). If your control valve goes fully open at 100 psig and resets to fully shut at some value under 100 psig then it is a relief valve and needs to be part of the safety valve calibration/testing plan. If it feathers open at 100 psig and tends to go shut at the end of the transient then it is a control valve and is subject to operational maintenance, not regulated maintenance.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist


That is a Pressure Relief Valve that you have.

Per ISO, only the term Safety Valve is used regardless of application or design.


A PSV is a national BPV code constructed / approved device (ASME OR BS or DIN or JIS etc), while a PCV or PRV isnt.


These are specialized liquid relief valves that are used on lube oil skids, at the discharge side of the lube oil pump. Unlike a typical relief valve, these valves are mechanically designed to open in a modulating manner (not pop-acting). Fulflo is a leading manufacturers of this type of valve.

In most cases these lube oil skids are servicing the bearings on machinery that is very expensive and very critical to the plant. Thus, the machinery needs a highly-reliable lube oil supply. For that reason, modulating relief valves are used in this service. A pop-acting relief valve could potentially cause a significant disturbance in the flow rate of oil to the bearing. By diverting the flow, a pop-valve could even starve the bearing of oil.

These are indeed relief valves, but since they are different from common relief valves, they are sometimes given different tag names. Maintenance/testing of these lube oil relief valves is commonly done at the same time that PM is performed on the skid itself, in which case these valves are not listed in the plant's overall PSV service program. That's another reason these lube oil relief valves are commonly given a different tag name. Doing so distinguishes them from the general population of plant PSVs.


Acceptable design practice in these applications, with minimal risk of an overpressure, is to install a spillback PCV / PRV with a PSV in parallel.

Without a PSV in parallel, risk of an overpressure is relatively higher, and the attendant consequences should be weighed up and checked for acceptance for this lube oil service case.


My take on this is that in "general usage" the terms "safety" and "relief" in terms of PSV / PRV conjure up occasional, non normal operation.

On the basis that your particular valves are in fact in continuous service / flow, then labeling them PCVs, whilst a little odd for a spring type valve is more in line with its mode of operation and I can see why this was changed.

If, on the other hand, these valve operate rarely, then Pressure Relief valve would seem to be a more accurate description of this valve

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close