11 replies to this topic

[gazepdapi1]

Can someone explain why typically 150# flanges are installed immediately on relief valve that will relieve at pressures might higher than a 150# flange can handle? I realize there is an orifice that cuts down the pressure but is the pressure cut that much? I've seen reliefs that will go off at 1400 psig (natural gas) and have 150# flanges on the outlet? Could someone explain this to me or reference some material I can look at?

Thank you

[paulhorth]

Nertil,

I realize there is an orifice that cuts down the pressure but is the pressure cut that much?

Yes, that's right, it is.
The pressure downstream of a relief valve is determined by the downstream system and piping, not by the upstream pressure. The downstream piping should be designed so that the pressure at the valve outlet at full flowrate is no more than 10% of the set pressure (for a conventional spring operated valve). This is a code requirement. So, for your 1400psi set point this would be 140 psi which is well within the limit of 150 # flanges.

Paul

Paul

Edited by paulhorth, 29 August 2012 - 04:43 PM.

[gazepdapi1]

ok thanks. do you know of any papers or literature that discusses this?

[Robert Montoya]

API 520 states that: Typically, the discharge system will be designed to ANSI
Class 150. However, higher flange class ratings may be required when the PRV discharge is to a closed system.
The reason is because the discharge pressure operating is around 30-50 psig hence the design pressure of the main header should not be higher that 100 psig

[gazepdapi1]

what section of api 520 is that stated in?

Nertil,

I realize there is an orifice that cuts down the pressure but is the pressure cut that much?

Yes, that's right, it is.
The pressure downstream of a relief valve is determined by the downstream system and piping, not by the upstream pressure. The downstream piping should be designed so that the pressure at the valve outlet at full flowrate is no more than 10% of the set pressure (for a conventional spring operated valve). This is a code requirement. So, for your 1400psi set point this would be 140 psi which is well within the limit of 150 # flanges.

Paul

Paul

what section of api 520 is that stated in?

How would this apply for a pilot operated valve?

Edited by nertil1, 29 August 2012 - 08:02 PM.

[Dacs]

How would this apply for a pilot operated valve?

Remember that a pilot operated valve is normally used for processes that expect a higher backpressure (around >50%) so your relieving pressure is not high in the 1st place.

Your flare stack is essentially atmospheric and any increase in pressure in your flare network comes from basically the backpressure of your flare discharge, which I doubt will require more than 150# rating.

[gazepdapi1]

How would this apply for a pilot operated valve?

Remember that a pilot operated valve is normally used for processes that expect a higher backpressure (around >50%) so your relieving pressure is not high in the 1st place.

Your flare stack is essentially atmospheric and any increase in pressure in your flare network comes from basically the backpressure of your flare discharge, which I doubt will require more than 150# rating.

Well, that's not necessarily true. We will use pilot valves to have tighter shutoff and a more controlled release (modulating pilot). The relieving pressure is still high (we have some pilot PRV that relieve at 2100 psig). These reliefs are actually vented to atmosphere

[Dacs]

Be as it may, even if you have a relieving pressure much higher than what 150# can support, it makes sense to design your downstream piping on the maximum pressure it will experience, and considering the nature of flare system (as I said in my previous post), I think 150# is enough in most cases.

As I quote Mr Paulhorth (bold for emphasis):

The pressure downstream of a relief valve is determined by the downstream system and piping, not by the upstream pressure. The downstream piping should be designed so that the pressure at the valve outlet at full flowrate is no more than 10% of the set pressure (for a conventional spring operated valve). This is a code requirement. So, for your 1400psi set point this would be 140 psi which is well within the limit of 150 # flanges.

Paul

The only way the discharge pressure of a relief will increase is if you experience an increase in backpressure, which will only come from other relief valves relieving (increase in destination pressure is out of the question if you're flaring to atmosphere).

Edited by Dacs, 29 August 2012 - 10:36 PM.

[fallah]

The only way the discharge pressure of a relief will increase is if you experience an increase in backpressure, which will only come from other relief valves relieving (increase in destination pressure is out of the question if you're flaring to atmosphere).

Increasing the back pressure in discharge line of a PSV (especially in conventional type), even excessive from allowable value, might also occur due to choking at the PSV outlet flange regardless of piping size, route and length.

Edited by fallah, 30 August 2012 - 12:40 AM.

[Dacs]

I agree with that, but I guess it might happen if the lateral is undersized to the point of having choked flow.

[fallah]

I agree with that, but I guess it might happen if the lateral is undersized to the point of having choked flow.

Not related, it can be occured even for zero tailpipe length.

[chemsac2]

PSV outlet flange rating is decided by set pressure, temperature and material of construction in accordence with API 526. For high set pressures, outlet flange ratings of 300 or even 600# are common.
However, flare piping apart from PSV mating flange is always 150# for economic reasons. This is not dictated by API. For given relief rate and flare tip pressure drop, we select flare piping diameter that results in 150# piping. Most of the times, flare headet mach number criteria automatically meets 150# criteria.

Regards,

Sachin