liquid seal overpressure protection
liquid seal overpressure protection
(OP)
Hi,
At our plant we have several tanks protected against overpressure by a liquid seal.
These liquid seals consist of a small pipe in another pipe with a larger diameter and a closed bottom end. (I've added a basic sketch for clarity)
In case of overpressure the liquid will move upwards and at a certain overpressure the gas will start flowing through the liquid .
Calculating the pressure at which the liquid seal opens and allows gas to flow through is rather straightforward but there have been some discussions about the scenario where a large amount of gas has to go through the liquid seal.
A larger flow means higher gas velocity through the liquid seal and possible fluid entrainment. In other words: two phase flow.
Does anyone have any suggestions in regards to calculating the increase in pressure drop over the liquid seal cause by this two phase flow?
(allowable gas velocity, liquid seal diameter,...)
Thanks in advance!
At our plant we have several tanks protected against overpressure by a liquid seal.
These liquid seals consist of a small pipe in another pipe with a larger diameter and a closed bottom end. (I've added a basic sketch for clarity)
In case of overpressure the liquid will move upwards and at a certain overpressure the gas will start flowing through the liquid .
Calculating the pressure at which the liquid seal opens and allows gas to flow through is rather straightforward but there have been some discussions about the scenario where a large amount of gas has to go through the liquid seal.
A larger flow means higher gas velocity through the liquid seal and possible fluid entrainment. In other words: two phase flow.
Does anyone have any suggestions in regards to calculating the increase in pressure drop over the liquid seal cause by this two phase flow?
(allowable gas velocity, liquid seal diameter,...)
Thanks in advance!





RE: liquid seal overpressure protection
I suggest you find another way to flow gas in the event of a relief scenario - PV valve, bursting disc, bent pin etc. All depends on what your unknown system is.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: liquid seal overpressure protection
As suggested, try a low dp backpressure control valve with low dp check valve; and
add an RD or buckling pin operated RV on a parallel bypass line across the backpressure PCV in case there are some operating scenarios that may be beyond the response capability of the backpressure PCV.
In some critical cases, you could add, on another parallel bypass line , an on-off quick acting gap acting fail open SDV that operates on PT feedback from upstream.
RE: liquid seal overpressure protection
I agree that some of the liquid seals will not be adequate as a measure against overpressure, depending on the scenario's, and will have to be replaced.
I was wondering though, up to which gas velocities can liquid seals be considered as a reliable overpressure protection? I've been looking for literature on the design of liquid seals but haven't found any good references so far. Do you know of any?
RE: liquid seal overpressure protection
None of the liquid seals are good for overpressure, not just "some"....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: liquid seal overpressure protection
RE: liquid seal overpressure protection
They have the advantage, or disadvantage, of also allowing vacuum relief. Of course in that case, you'll draw a slug of the liquid in the liquid seal back into the tank, so it must be selected to make that safe.
In the case that the flow through the seal is too high, it will simply blow the liquid out and become an open connection to atmosphere. If back-flow of atmospheric gas after the relief event is unsafe, then indeed these units are unsafe for that application- but then, a rupture disc would also be unsafe.
If you select the wrong MOC, the wrong liquid for use in the seal etc., of course they won't work. But as a means to protect a small atmospheric tank which needs a small pad pressure for inerting etc., they're better than a lot of other options you might select.
RE: liquid seal overpressure protection
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: liquid seal overpressure protection
LittleInch, most of these liquid seals are equipped with a level detection which detects when the liquid seal is blown out and generates an alarm(sometimes before the seal is blown).
Measures have also been taken to ensure that the liquid seal is full.
So during normal production the liquid seal will be full, and when the seal is blown because of high pressure this will be detected. In some cases this automatically shuts down production.
RE: liquid seal overpressure protection
what sort of inches water column are you working with?
Are they all separate seal pots or are some roof sleeves?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: liquid seal overpressure protection
In one instance we're working with 100 inches water column (250mbar)
I'm afraid I am not familiar with the term roof sleeve, what do you mean by this?
RE: liquid seal overpressure protection
gluedideas.com/Encyclopedia-Britannica-Volume-10-Part-1-Game-Gun-Metal/Gasholders.html
A pot is a better idea especially if you have level switches to tell you when the seal is gone.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: liquid seal overpressure protection
The problem right now is that HAZOPs usually require proof that these liquid seals prevent the tank from being pressurised above its design pressure. At low flow this is not a problem as the gas will bubble through the liquid seal at its "set pressure".
For some of these liquid seals however, we've found scenario's where the flow through the liquid seal seems high enough to blow out the liquid seal and creating a higher pressure drop during the process.
As I understand it, the next steps would be:
any advice (preferably on the first step)?
RE: liquid seal overpressure protection
As to step 1: ensure that the outlet piping and seal chamber can pass the relief/overpressure scenario flow without overpressuring the tank in the absence of the fill liquid, i.e. when the pot is empty. If it passes that test, you're only worried about a brief transient state when there is two phase flow of relieving gas and liquid from the seal pot in the discharge line. If you're close to the limit without the liquid, you'll be over the limit with the liquid and you need to start designing plan B. If you're well below the design pressure without the liquid, a brief spell of entrained liquid likely isn't going to push you over. But if you're close, you'll have to make an assumption about the rate at which the liquid leaves- assume some period of time for the liquid to leave, say 1 minute, and use the DIERS method or a conventional 2 phase flow method from fluid mechanics to calculate that brief, peak overpressure. A properly designed liquid seal will never have its exit tube completely filled with liquid, so the flow of pure liquid isn't a case you need to consider. If that brief peak pressure is above 110% of MAWP but below the hydrotest pressure for the tank, you'll have to decide whether or not a retrofit is necessary, since you should be reasonably confident that exceeding 110% of the MAWP for a few seconds by that small margin is unlikely to cause the tank to rupture. Depending on the contents and size of the tank, you can decide just how conservative you need to be.
RE: liquid seal overpressure protection
RE: liquid seal overpressure protection
For the last of these, there are some recommendations in Perry for the design of sparger pipes in the liquid - gas chapter. Briefly it says that superficial gas velocity through the cross sectional area of the seal drum should not exceed 0.2ft/sec for non turbulent flow. Else another solution may be a larger seal drum?
RE: liquid seal overpressure protection
The following design features from "PRESSURE SAFETY DESIGN PRACTICES FOR REFINERY AND CHEMICAL OPERATIONS" are usually incorporated for liquid seal:
1. Continuous water makeup and overflow on the seal loop, to ensure that
the seal is always made during normal operation, and reestablished after a blow.
2. Adequate winterizing, where necessary, to prevent freezing of the seal.
3. Safe disposal of the effluent seal water, considering possible contamination by process fluids.
4. Specific criteria which govern the acceptability of discharging process fluids to atmosphere.
5. Contingencies by which liquid hydrocarbon could be discharged through the atmospheric vent must be positively eliminated.
6. The vent line must comply with safe practices to prevent flashback and snuffing requirements