Relief valve

[lorenzo67]

I've a doubt.
Is possible to insert an RO after a relief
valve? If it isn't how can I reduce the pressure?
Thanks

 

[davefitz]

No , you cannot add an RO- there already exists an RO in the relief valve. The rated capacity is based on the installed orifice in the relief valve and the neccesity that the downstream pressure be less than 45% of the set pressure ( for most gaseous relief valves with open drip pans in the relief stack) .

The pressure drops by means of the relationship between W/A vs P for acoustically choked flow. First , the max relief flow W is related to the component with the minimum flow area, which is supposed to be the relief orifice. Once that flow W is set, then all of the downstream pressures are set by the nearly linear relationship between P and W/A for isenthalpic flow at the speed of sound. The exceptions to this may be for very long exhaust stacks that may be frictionally choked.

See ASME B31.1 appendix II or the Bechtel Liao method of calculating flows , pressures, and reactions is discharge piping.

 

[lorenzo67]

I've a relief valve on liquid is it the same?

 

[MJCronin]

Relief valves are typically installed in systems where the maximum pressure must be limited.

When the design pressure of the system is exceeded, then the protected system should be relieved of pressure "as quickly as possible".....especially in systems where further pressure build-up is likely.

The concept: "as quickly as possible" governs the mechanical design of the the relief valve.

Putting an RO "after a relief valve works against the "as quickly as possible" principal.

If the rate of pressure relief must be controlled, use a control valve suited for that purpose. Huge utility boilers have such a valve on thier main steam systems......called PORVs (power operated relief valves) The pressure drop across these valves is tremendous, but the operator can control the rate of relief and therby keep the boiler under control.

My opinion only

-MJC

 

[rneill]

Code clearly prohibits flow restrictions in discharge lines from relief valves. The pressure drop at the discharge of a relief valve is purely the result of the built up back pressure from all the elements in the downstream piping (the flare tip, the flare stack, and the piping).

If you need to reduce the pressure downstream of the relief valve, the only way to do this is to reduce the pressure drop. Install a different flare tip, a larger diameter flare stack, or larger diameter flare discharge piping.

With regards to the suggestion to install a control valve to limit the rate of relief, this could only be used in parallel to the relief valve (perhaps with a lower set point). Code does not permit such installations to replace the installation of a conventional pressure relief device. Also, such control will only work if the actual rate of pressure rise is less than the ability of the valve to relieve under controlled operation. If the valve is not relieving enough, and the pressure relief valve set point is hit, then the relief valve will lift and you will have the full flow through the discharge piping again putting you back where you started.

I would ensure that the discharge piping was sufficiently large or rated such that during the design relieving event, the downstream piping was capable of the full backpressure imposed.

 

[JohnGP]

lorenzo67,
If you provided more information as to why the downstream pressure needs to be reduced, then a bit more focus could be applied to the actual problem, rather than guessing what you are trying to achieve.
Cheers,
John

 

[lorenzo67]

Yes, John I try to explain the question.
I've a volumetric pump and I want protect the discharge line from over pressure that can occur when the valve on the line is close. I have to send the discharge of relief valve in a atmospheric tank. The set point of relief valve is 10 barg.
How I can do that?

 

[ione]

When pressure upstream the relief valve reaches the set threshold the valve opens and discharge to an atmospheric tank (0 barg). So what’s the matter? Just size properly the line downstream the relief valve to the tank, and take into account back pressure of this line.

 

[lorenzo67]

Ok. Ione
But, this pump is near the atmospheric tank.
Do you think is a problem or not?

 

[ione]

If you discharge in the bottom of the tank, when you compute back pressure you should take into account the static head of the fluid present in your tank. You could use a rubber joint in order to dampen vibration or shock waves.

Just my opinion

 

[MortenA]

Dont put an RO upsteam or downstream a PRV - its counter productive!

besides - if your upstream rating is higher than your downstream rating you risk over pressuring ypur downstream side.

Best regards

Morten

 

[lorenzo67]

I'm agree whit you ione but the vessel is empty usually so it is the same.

 

[JohnGP]

lorenzo67,
I'm still having a problem seeing why you need an RO in the discharge line of the relief valve. You naturally get a drop in pressure through the PRV, to a backpressure value that is determined by the diameter and length of the line from the PRV, along with the flowrate.

Normally you would direct the PRV discharge to the top of the atmospheric tank, but as ione points out, the static head in the tank will add to the backpressure on the PRV if it discharges to the bottom of the tank. (I would direct it to the top of the tank.)

Cheers,
John