Relief Valve Capacity Derated by Inlet Piping 3

[Feedyourhead]

So we have engineering software that is capable of calculating the effect that the piping has on a relief valve. But how is it doing it? What is engineering theory that would provide an analytical solution to how much a relief valve is affected by pressure loss?

The typical relief valve situation for my application is as follows:

A section of pipe T's off from the system to be protected and has a relief valve at the end. The piping from the T to the relief valve is considered the inlet piping. There is also outlet piping after the relief valve typically straight up like a chimney.

From what I understand is that engineering code recommends no more than 3% pressure loss on the inlet to prevent chattering, and no more than 10% pressure loss on the outlet to prevent too much backpressure. But what about derating the actual capacity of the relief valve with the pressure losses? How is it calculated? How is my software calculating it? Say for example I am able to calculate that there will be a pressure loss of 5 psi in the inlet piping, and that the relief valve is able to pass 400,000 cubic feet per hour at full capacity. What is the new capacity now that there is a 5 psi pressure loss in the inlet piping because it is 100 feet long?

 

[georgeverghese]

Indeed, ASME B31.8 does permit the use of pilot op relief valves for overpressure protection as an alternate to ASME coded safety valves

"845.2 Control and Limiting of Gas Pressure
845.21 Control and limiting of Gas Pressure in Holders, Pipelines, and All Facilities That Might at Times Be Bottle Tight
845.212 Suitable types of protective devices to prevent overpressuring of such facilities include
(a) spring-loaded relief valves of types meeting the provisions of BPV Code, Section VIII
(b) pilot-loaded back-pressure regulators used as relief valves, so designed that failure of the pilot system or control lines will cause the regulator to open
(c) rupture disks of the type meeting the provisions of BPV Code, Section VIII, Division 1"

 

[shvet]

Yes trying to avoid undercapacity caused by excessive inlet piping

Recommendation known as "3% rule" is included in API 520-2 only and applies to PSVs having design the same as shown in API 520-2. You mixed up function with design. Control valves your type of have similar function as pressure relief devices and dissimilar design. This means that there is no evidence that there is a risk a pilot backpressure regulators exhibits chattering.

Your question is not clear. You have asked how capacity decreasing should be calculated and obtained replies that you shall calculate actual pressure drop and fluid properties. Calculation of mass flux of a control valve does not differ from any other control valve - you need to find pressure drop and properties at inlet&outlet conditions, nothing more or less.

What is your question now?

Is "3% rule" applied to NG piping?
Answer - is not, as:
- "3% rule" is not a rule, it is an advise
- "3% rules" relates to equipment overpressure, not piping
- your piping code does not contain "3% rule"

How is your software calculating a capacity of this pressure safety valve?
Answer - nobody knows. This software is yours and forummembers know nothing about it.

 

[Feedyourhead]

Yes maybe I'll have to read more about the 3% rule. If it is true that the omly factors that affect capacity are inlet and outlet pressure, and the inlet piping pressure loss only affects overpressure, then it is likely that the reduced capacity that my program was seeing may be from the isolation valve on the inlet piping. It is possible that the isolation valve has less capacity than the relief.

 

[1503-44]

The isolation valve on a relief is not a great safety idea, but thet can be practical for maintenance purposes. Be sure it is Car Seal Open. It should also be bored to full ID of the relief pipe,

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[davefitz]

The pressure drop from the T to the relief valve inlet must be less than the differnce between the lift pressure and the "blowdown" pressure, or else it will lead to chatter and grossly reduced actual relief flow.

The flow rating should be adjusted to reflect the lower inlet pressure to the relief valve compared to the protected vessel or piping. For gaseous releif valves, nearly all cases assume acoustic choked flow at the valve's orifice and credited as about 90% of the tested or calculated choked flow, and since choked flow is linearly proportional to the inlet's absolute pressure, one must adjust the credited relief flowbased on the pressure loss in the inlet pipe.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick