nh3 pressure relief valve lifting

[barnstable1197]

I'm trying to identify relief valve lifting problems. If an nh3 pressure vessel's inlet valve leaks by and the vessel fills completely, should the relief valve lift if the incoming pressure is below the lifting pressure of the relief valve?
thanks

[RANGER1]

barnstable1197
If the pressure is below relief valve setting it should not relieve but ...

relief valves are only meant to relieve gas only not liquid .

What pressure is relief set at & when were they last changed or recalibrated ?
Also explain circumstances in more detail .

[barnstable1197]

barnstable1197
If the pressure is below relief valve setting it should not relieve but ...

relief valves are only meant to relieve gas only not liquid .

What pressure is relief set at & when were they last changed or recalibrated ?
Also explain circumstances in more detail .

relief valve setting is 150 psi on nh3 expansion tank. liquid supply is at 122 psi. if the gas outlet valve closes and the unit fills with liquid shouldn't the pressure only get as high as 122 or will it climb higher. there are no heat sources that would get the temp of the liquid past(the room it cools is at 60 degrees max which is 92 psig on a nh3 press temp chart) 150 psi. with the unit full of liquid would a 5 or 10 ten gegree in external temperature be enough to cause the liquid to expand and lift the relief valve?
The relief are dual and are new and calibrated.

[RANGER1]

If the vessel ( tank ) was near a 100% full & any temp increase would lift relief due to hydraulic effect , expansion of liquid.

Why does it fill up vessel?

[Grizzly]

relief valve setting is 150 psi on nh3 expansion tank. liquid supply is at 122 psi. if the gas outlet valve closes and the unit fills with liquid shouldn't the pressure only get as high as 122 or will it climb higher. there are no heat sources that would get the temp of the liquid past(the room it cools is at 60 degrees max which is 92 psig on a nh3 press temp chart) 150 psi. with the unit full of liquid would a 5 or 10 ten gegree in external temperature be enough to cause the liquid to expand and lift the relief valve?
The relief are dual and are new and calibrated.

Why are the relief vaves set so low?
Grizzly

[RANGER1]

True Grizzly , as thats only 1050kpa .
Need to check pressure vessel details to see maximum working pressure then select relief to suit .
We try to keep things simple by setting liquid reciever etc @ 1700 kpa = 250psi & low side vessels @ 1150 kpa = 160psi
Depending some high temp rooms would have 1700kpa reliefs .

[Magoo]

I agree with RANGER.
The vessel should have design criteria name plate. With minmax temps. Generally here they are rated for 1.72 Mpa.

[priti]

If the vessel is 100% full with refrigerant liquid (any refrigerant) and there is no more space for liquid to expand in responce to little increase in surrounding temperature; it will cause to increase pressure inside the vessel.

In such case; if the pressure will reach the relief valve setting, the safety relief valve should relieve the excess pressure.

And of course, if the relief valve is not there, it may reach to an extent of rupture of vessel. (please refer refrigeration handbook by ashrae)

regards.

Priti

[Grizzly]

If the vessel is 100% full with refrigerant liquid (any refrigerant) and there is no more space for liquid to expand in responce to little increase in surrounding temperature; it will cause to increase pressure inside the vessel.

In such case; if the pressure will reach the relief valve setting, the safety relief valve should relieve the excess pressure.

And of course, if the relief valve is not there, it may reach to an extent of rupture of vessel. (please refer refrigeration handbook by ashrae)

regards.

Priti

No-one is saying there should not be a relief valve.
Nor that sufficient pressure to lift one could exist.
Some , myself included are asking why.
The existing valve is set so low?
Grizzly

[McFranklin]

Why are the relief vaves set so low?
Grizzly

I hate to admit it, as late as '98 I was quoting vessels rated at 150# with an adder to have it rated at 250#. It might save you a $1000 on a bid.
The market was that tough.
The manufacturers more or less put an end to the practise by settling on 250# as a de facto standard.

[NH34ICECREAM]

Match the relif valve to the MWAP of the Vessel , also try to install a rupture disk with the relief valve
This could help with weaping of the relief .

[That's too cold]

If the liquid is filling above the relief valve at 122psi, the hydraulic shock will lift it. Alot of good advice is given above. Here is a link to Hansen Rupture Disks http://www.hantech.com/library/pdfs/...lief/K209C.pdf

[sterl]

The key point is that once the relief pressure has been reached the valve will permit flow until the vessel pressure reduces to the reseating pressure. The difference is called blowdown: So even though it may take only a quart of flow to reduce the vessel pressure to below 150, it can take considerable flow to get the valve to its reseating pressure.

Is the relief actually activating or just "jumping off" and reseating almost immediately? Is there a long pause after the relief valve re-seats before it pops off a second time; or does it pop off for a brief period and reseat, then not pop off for a week or more?

The blow down on the available range of "normal" relief valves varies from 10 to as much as 35%. Note that the ratio of your source pressure to your RV setting is over 80%. If you have an isolation valve on the suction and a check valve on the liquid, and no other way for the refrigerant to escape, and its possible for the vessel and refrigerant temperature to exceed 82 deg F. (saturation at 150) you could have a vapor space in the vessel and still pop the relief valve. If your temperature can only rise by a few degrees you can still make 150-psi plus with makeup of 112 psi liquid pretty easily: but the vessel would first have to fill to near 100% liquid; then warm up after its isolated. It sounds simple but actually getting a vessel to 100% full is more difficult than it sounds unless the liquid source is subcooled compared to its delivery pressure.

So an RV with a blowdown of 20% or larger is going to do one of 3-things: in response to a small shock its going to weep for a half-second and reseat; in response to a true rise in standing pressure it would open and tend to stay open until the pressure drop in the liquid line reduced the source pressure past the blowdown envelope and then the valve would reseat...which would happen reasonably quickly with a 10% blowdown valve and would continue a long time should the valve have a 35% blowdown.

If this was a temperature rise scenario the relief valve's flow would normally cause a continuing reduction in vessel pressure, which would cool the liquid and also introduce a vapor space; ...So for a valve that did reseat, there would be a long reseated period while the vessel filled and subsequently warmed back up. You might then have another release...That might take many hours. BUT.... To get the temperature gain to increase the pressure over that of the liquid source: both the suction and the liquid need to be isolated. If there is a check valve on the liquid side this is possible but there is largely no way for the vessel to reach a pressure higher than the highest pressure it is connected to unless there is means of isolating that line. This a transfer of some sort or a liquid recirc arrangement with HG defrost or ???

If its the response to a large shock then it will open, and flow for a minimal period of time; and providing the vessel pressure reduces to below the blowdown envelope: it will reseat, and stay that way until the next shock event. But once opened, the valve will continue to flow until the vessel pressure has been reduced by the blowdown. SO....If it was a 35% blowdown valve, for instance; considering the 112 Psi source still making up to the vessel: most of the charge could go out the relief line before the valve reseated because it would not want to reseat until the vessel pressure dropped below about 97 psi; and your liquid line might be able to ensure the vessel stays above that pressure depending on how its flow capacity compares with the flow capacity of the safety; and whether or not the safety is passing vapor or liquid.

SO: Sort it out...Ask the manufacturer for the valve's blowdown. If its high and you still have a suitable quantity of refrigerant, you almost have to think it was the "minor" shock. But if there is no check valve and quite a bit of refrigerant is gone: Major shock most likely. If its got a means of isolation from the liquid source: Well it could either be shock or temperature rise; but if it also repeats flowing, the temperature rise would be more realistic. With a low blowdown: the valve should reseat almost immediately after a shock; but will probably take some time to reseat if the vessel and contents go through the temperature change as well.

So: Which is it?

[Terron]

Typically, vessels containing nh3 should have safety measures in place to prevent over filling of the vessels.There should always be room for expansion of the ammonia, 20% from the top is usually left for vapor or gas. Unless, it is an oil pot on a vessel or an oil cooler on a compressor. Any vessel that has the potential to relieve liquid needs a pop-off or relief valve assigned for that duty to ensure hydraulic lifting does not take place. 150# relief valves are typically used on the low side since most compressors and vessels on the low side should never see 150# when running properly. I run a two stage system and my boosters discharge pressures are always below 35#'s. The vessels and compressors on the low side are all rated for 150#'s. My question to you is, "why are you filling your vessel 100%)?