Backpressure on Safety Relief Valves 1

[kvbalu2129]

Hi all

I am designing a Safety valve relief outlet piping handling saturated steam at 270 Deg.c. Set pressure of the valve is 50.0 kg/sqcm and the size of the safety valve is 4 x P x 6. Required Relieving capacity of the valve is 91 tph. The valve has been already selected and to prevent a bellows safety valve, the consultant had specified a built-up back pressure of 5 kg/sqcm(g), which is 10% of set pressure. The problem now is about the size of the relief piping. A 6"line of 3 Metres would relieve only about 53 tph while 8"line(after providing an expander at valve outlet) would provide 86 tph, to maintain the backpressure below 10% of set pressure.Any size increase above 8"does not increase flow ( to provide atleast 90 tph)

Would request some tips on how to provide the required minimum relieving capacity in the outlet piping without changing the safety valve.

 

[MortenA]

Why dosnt a further increase in size increase the capacity?

 

[kvbalu2129]

Flow (Velocity) is choked when the downstream pressure falls below the critical pressure. Mass flow rate can be increased only by increasing density, i.e. pressure upstream of the choked point.

 

[Latexman]

My software said 8" had too much backpressure too, but that 10" is okay. Exactly where is flow choked?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[kvbalu2129]

Thanx Latexman. Flow chokes at Relief Valve & at Exit to atmosphere. Any change of size above 8" wouldnt remove the choke.

 

[kvbalu2129]

In addition, any area change (reducers) would choke velocity. Kindly add your views.

 

[Latexman]

I understand choking "at Exit to atmosphere". What do you mean "Flow chokes at Relief Valve"? At the exit of the "P" nozzle, at the outlet flange, where?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[kvbalu2129]

Choked flow at the nozzle. Nothing can be done about it any way as the Relief Valve orifice is fixed.

regards

 

[Latexman]

Try a 6" tailpipe of zero length and no fitting losses. I think you have "body choke" and the choked flow pressure exceeds the 10% backpressure limit of the PSV.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[kvbalu2129]

Sure latexman. Thanq for your tip. Wil get back to the forum

 

[kvbalu2129]

But wouldnt choked flow of the Relief valve be the value of 98TPH provided by the Relief valve vendor? I looked up at the sizing datasheet of the relief valve for the value (Actual Relieving capacity of Valve). Have I got it all wrong?

 

[Latexman]

This is a known dilemma in the relief design world.

Reference: Guidelines For Pressure Relief and Effluent Handling Systems, Center For Chemical Process Safety of the American Institute Of Chemical Engineers, 1998, pages 38-39. “On certain occasions, the designer can encounter a dilemma when sizing a tail pipe to meet the built-up back pressure limitation for conventional pressure relief valves. Calculations for a tail pipe of the same size as the outlet flange can say that the built-up back pressure exceeds the allowable limit even if the length is reduced to zero.”

For gas flow, this problem usually arises only for valves with small ratios of outlet-to-nozzle area and rather high set points.

Using this case as an example, an Engineer sizing a 4P6 PSV with no tailpipe sees no problem because there is no need to calculate backpressure.

An Engineer sizing a 4P6 PSV with tailpipe sees the problem because they must calculate backpressure.

The problem does not arise unless the calculated flow from the valve nozzle is predicted to be high enough to attain choked-flow conditions in the pipe size of the PSV’s outlet flange. But, choking is not the problem. The problem arises only if the choked-flow pressure exceeds the allowable back pressure limit for the particular valve, such as this case.

The problem can appear to go away if a larger tail pipe size is specified without proper modeling of the flow in the pipe expansion. The dilemma remains nevertheless - if the back pressure is too high even with zero length of outlet size pipe, the back pressure will not be reduced by addition of pipe of larger size, right?

The dilemma then is whether or not to change the PSV to obtain calculated results in accord with the non-mandatory Code requirement (select a valve or valves with higher outlet flow area for the given nozzle flow area). And, in this particular case, I don’t think a P nozzle PSV with a higher outlet flow area exists. It is quite a dilemma.

So, what to do? Definitely, work with the vendor’s “expert” or technical support. They have seen this issue before. Can you eliminate the tail pipe? Can you reduce the set pressure to the point where the backpressure equals the accumulation? (I calculated about 12% backpressure). Can you use two smaller PSVs? Can you modify/change to bellows PSV? Can you use a Pilot PSV?

Kvbalu2129, you have a very interesting problem here. Please come bach with more questions and let us know how it is resolved.


Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[don1980]

For these API 526 valves, it's easy to understand this problem when you look at how the orifice area increases disproportionately to the outlet flange area. The valve body flanges (open area) don't increase in size anywhere near as much as the increases in area for the orifices. This is a widely recognized problem. The best solution is to turn the clock back and re-write the original API 526 standard (e.g. 6P8 or 8P10 instead of 4P6). That obviously can't be done, so we're stuck with making the best of what we have.

One solution is to use a restricted-lift PSV. In other words, buy a 4P6 valve with a reduced capacity - a capacity that matches the capacity that you need (required capacity) instead of the full capacity of a P orifice. I know that at least one of the major manufacturers in the US can do this, and provide ASME code certification for the valve. A restricted-lift valve has a mechanical stop (collar) that fits around the stem, limiting the lift and effective orifice area. This results in a lower rated flowrate for the valve. Since pressure drop increases square to the flowrate, it doesn't take much of a flow reduction to get a big reduction in pressure drop.

 

[e43u8]

Also an alternate solution can be using a pilot operated PSV whose sensitivity to the back pressure is much lower...

 

[kvbalu2129]

Hi all

I think a 10" tailpipe would do the job (an expander 6"x 10" at the relief valve outlet matching flange) and the back pressure would be around 4.8 kg/sqcm. Higher sizes than 10" wouldnt make any more difference. and Pray that the valve would never have to pop!!! ThanQ all, specifically Latexman and don1980. I am now working on estimating a Cv for a relief valve.