5 replies to this topic

[tok]

Dear all
Please advice me,Is there any standard for how distance should be in inlet of PRV,
Can we install PRV far from equipment ?
Thanks
tok

[kdal]

Refer to API-521.
For a conventional pressure relief valve the pressure loss in the inlet piping/fittings should not exceed 3% of the valve set pressure. The length of piping is really irrelevant so long as you keep to this rule.
K

[proinwv]

Yes remember that the relief device will react to the pressure at it's inlet.

If there is no flow in the line the pressure is the same throughout. Once flow commences, the pressure degrades in the line due to friction and the pressure at the relief is LESS than the pressure at the source.

You must be aware of this DP and be sure that it will not effect safe operation.

Best solution is to always place the relief where you want to control the pressure.

PAUL

[pleckner]

Firt, let me start off by saying the answer to your original question is a combination of both of the previous answers.

Now to answer the question on the 3% rule. All relief valves (except the pilot versions) have what is called a blowdown pressure associated with them. This is the pressure that must be achieved to allow the valve to close and fully reseat after is has opened. Upstream system pressure greater than this value will keep the valve opened.

Many years ago, this blowdown pressure was, for the most part, pretty low compared to the set pressure. For example, a PSV with a set pressure of 50 psig might have had a blowdown of about 5 psig or even less. (Some had higher values. I worked on one system where an existing PSV had a blowdown pressure of 16% of set point). It was very much a function of the mechanical construction of the valve, therefore, it was very manufacturer and model specific. Also, these blowdown pressures were preset in the factory and were fixed. The significance of all this is that if during relief, the pressure at the relief valve inlet dropped below this blowdown, the valve would close. The system pressue would then increase and the valve open again. This would cycle (and happen very fast) until the relieving condition ended. This is not good for the mechanical intergrity of the valve.

So, the ASME committee came up with this guideline/recommendation (note, not mandatory nor law) that to ensure the valve is kept opened during relief, the amount of pressure drop from the relieving source to the PSV needs to be limited. What to make it was the $64,000 question. They came up with 3% of set pressure because that seemed to be far enough away (conservative) from the lowest blowdown seen in the industry.

Why would the pressure at the valve drop below blowdown during a relief? If the flow of the relieving fluid was too great for the inlet line size, too long of a line, too many fittings in the line, having to travel through equipment.....that would do it.

Now, here is where I have a problem with the rule. The rule only refers to non-recoverable pressure losses and basically that means flowing frictional losses only. It also does not distinguish between gases/vapors and liquids. We all know that height can have a big affect on the pressure in a system when it concerns liquids, i.e. static head. So, if I have to go uphill to reach a relief valve, then I loose pressure from source to the valve not only due to flowing frictional losses but also due to static head. And, this static head is NOT required to be counted in the calculation; a mistake as far as I'm concerned.

[kdal]

totally agree with Phil's response. I would like to add that API-520 part II does allow higher than 3% inlet losses IF an engineering analysis is conducted. This relaxation comes from the nature of the 3% rule as Phil describes. The 3% is somewhat arbitrary and it may well be that the specific valve in question can in fact operate without chattering with inlet pressure drops greater than 3% of set pressure. It's always worth contacting the manufacturer if you have a situation where you are forced to consider higher inlet pressure drops.