Inlet Pressure Drop for Emergency Relief Valve

[Rhy8]

Hello, I would like to know if the 3% rule still applies for pressure relief vent protecting a tank under code API 2000. I have a filter weigh bin with pressure relief vent set at 0.625 psig (MAWP of equipment is 1psig). As per vendor (i.e. GROTH), this valve opens fully at 100% overpressure (OP) (i.e. double the set pressure). However, 10% overpressure is enough to cover the required capacity. Since my fluid is vapor, if I consider 100%OP, I have to used the rated capacity of the valve at 100%OP which results to more than 3% inlet presssure drop because of the entrance loss from te filter to the pressure relief vent valve. Actually, even the 10%OP gives me more than 3% inlet pressure drop. My question is, I wonder if the 3% rule is applicable for this valve. Valve is model # 2300A Groth vendor. The blowdown for this valve is 8%. I am worried that I chattering may occur if I have more than half percent of the blowdown. At the same time, I am not sure if the 3% rule for ASME coded equipment is applicable for API2000 vessel. Thanks and I appreciate your input.

 

[TD2K]

No.

API 650 tanks have no overpressure allowance. If the tank is designed for 1 psig, your relief device must be set low enough so that it passes the required volume without exceeding the design pressure. API 620 tanks are allowed a 10% overpressure but I don't recall seeing any inlet line loss criteria for either tank specification.

The 3% for ASME code vessels was based on historical blowdown settings being in about the 7% range. By limiting the inlet line loss to 3% or less, you avoided chattering in the relief valve due to excessive inlet line losses (you can still get chattering with a properly sized inlet line if the relief valve is oversized for the vapor flow to be relieved).

Interesting, I don't recall blowdown % on any tank vent valves I've looked at in the past.

You know the capacity of your Groth valve and you know the vapor flow to be relieved. If that only requires 10% overpressure when the valve is set at 0.625 psig and the tank design pressure is 1 psig, your are fine. Why are you looking at the hydraulics at 100% overpressure? Are you thinking along the line of PSV's inlet and outlet piping being evaluated at the relief valve's capacity?

 

[Rhy8]

Thanks very much for the response. This is very helpful. I didn't know if the 3% rule applies for this because the vendor gave me a percent blowdown. Yes, I was thinking about the inlet piping being evaluated at the relief valve's capacity. The hydraulics shows a very high pressure drop because of the entrance from the filter to the small piece of pipe before it gets to the PSV. I think the reason why we don't consider that is maybe because it is already included when they size the valve at the beginning???

 

[don1980]

The 3% inlet loss limitation just applies to pop valves, and it's not at all applicable to weight-loaded tank vent devices. These vent devices have no risk of chatter. They're inherently modulating devices.

Piping pressure drop can't cause these devices to chatter, but it can result in the tank exceeding its max allowable pressure. If the device is mounted directly on the vesel nozzle, then piping pressure drop isn't significant and can be ignored. If you have piping between the vessel and the device, or any significant backpressure cause by pipeing, scrubber, etc, then that pressure drop can result in the vessel exceeding it's allowable pressure. The set pressure needs to be adjusted to account for that.

 

[Rhy8]

Thanks Don1980 for the response. The pipe is only 0.5 ft which gives a very small pressure drop. The fitting that gives a very big pressure drop is the entrance from 7ft diameter of the filter to 4 in pipe diameter). Unless the entrance should not be accomodated on pressure drop calculations for weigh loaded relief valves?

 

[Latexman]

The entrance should be included. Usually the static pressure in the vessel is known (or assumed). This is used along with the pressure drops along the flow path (entrance, pipe and fittings) to calculate the pressure at the inlet of the pressure relief vent.

Good luck,
Latexman

 

[TD2K]

Definitely include that entrance loss.

 

[Rhy8]

Thank you. My guess is that they have already taken into consideration the losses when they sized the valve. The loss due to entrance with the required flow is 0.13 psi. The MAWP of the equipment is 1 psig and the set pressure is 0.625 psig. Thus, I believe they already accomodated this drop in setting the set pressure. Thank you all for the input. This is my first post here and I thank you all for the answers. I now finalized my calculation.

 

[Latexman]

Is it an end-of-the-line or pipe-away pressure relief vent?

Good luck,
Latexman

 

[Rhy8]

Actually, it starts from the filter nozzle, then 0.5 ft pipe then the PSV valve. So, not sure if that is a pipe away relief vent because it's only 0.5 ft pipe. Thanks!

 

[Latexman]

A pipe-away has an outlet line.

Good luck,
Latexman

 

[Rhy8]

Oh I see. There is no outlet line. The pellet is lifted until it relieves all the required flow and then reseats back again. The relief goes to the atmosphere. It's weighted relief valve. But I think it's ok now. From the info I gathered, the 3% rule does not apply. Even if I will consider the 0.13 psi drop at the inlet, I am still below the MAWP of 1 psig (set pressure 0.625 psig). My main worry before was the 3% rule which I believe is not applicable for weighted relief valve. I do agree with don1980.

 

[Latexman]

You are okay.

Good luck,
Latexman

 

[Rhy8]

Thank you all for your time.