7 replies to this topic

[teedot]

The company's design standard requires a depressuring system (to be designed as per API521) to be installed on PROCESS VESSELS that

1. contain flammables with true vapor pressure greater than 2 bar @ 54 dec C
AND
2. vessel is designed for pressures equal or greater than 30 barg

The standard seems to be very broad and would I like to know if a system with the following parameters would make use of such a depressuring system or would it be useless:

1. Subject vessel is liquid filled
2. Vessel OPERATING pressure is only 11 barg
3. Vessel DESIGN pressure is 32 barg due to pump shutoff pressure
4. The liquid's vapor pressure @ operating temp is below 6.9 barg, which is the required pressure to be reached within 15 min as per API-521 ISO23251.

Any comments will be highly appreciated

[Zauberberg]

Every pressure vessel must be equipped with a pressure relief device, and this requirement is determined by the law. Full stop. A company standard certainly cannot be taken as the ultimate regulatory act.

[paulhorth]

Teedot,

Zauberberg is of course correct that your vessel must have a pressure relief device ( PSV). But I believe you are asking about a depressuring system, which is different. A PSV does not depressure the vessel. A depressuring valve (blowdown valve) will open and stay open to allow the pressure to fall.

In the case you describe, the vessel conditions fall within the company equirements for a depressuring device to be fitted, so yes, you should fit a depressuring valve. If the vessel is indeed full of liquid, this valve will open and pass only a small amount of liquid, which should be routed to the flare system. This function will protect the vessel from overpressure if exposed to a fire. You have to assume that the vessel is at its design pressure of 32 barg when a fire occurs and the blowdown is called on to act.

Without a blowdown valve, if the vessel is exposed to fire and blocked in, the PSV will open but the vessel will continue to rise in temperature until the contents start to boil. at the PSV set pressure. The PSV should be sized for this vaporisation rate. However, as the liquid boils off, the upper vessel walls will not be cooled by the liquid and will get hotter, and it is possible that the vessel will rupture leading to a BLEVE (Boiling Liquid Expanding Vapour Explosion). In the past such BLEVEs have caused many deaths.

Paul

[fallah]

For vessels that can be isolated and are exposed to fire:

1-If it is full of HC liquid no need to BDV installation.

2-If it is full of liquified HC (containing more than 2 tons of C4 and more volatile),BDV installation is required

[bernath]

Dear Paul Horth,

A very nice answer from you, It was very enlightening. I've got a few questions to add though.

I agree with it. A PSV doesn't depressure the vessel. Blowdown valve does it. IMHO the reason is due to the time needed for PSV to depressurize the vessel. PSV has to be on set pressure plus 10% over pressure condition to be fully opened while blowdown valve can be opened at anytime by operators. There might not be enough time for PSV to depressurize the vessel, therefore even though the vessel is equipped by PSV, it can still be failed if the pressure is not released quickly enough. Thus it' very crucial to add a blowdown valve to this system. This is my own understanding. Please clarify if I'm wrong.

My question is how to determine the blowdown load? I know how to calculate the relief load for PSV due to overpressure, but I'm still not clear about the blow down and depressuring load. Does the term of blow down and depressuring refer to the same meaning?

For depressuring system, I have found one good statement across the net. "When metal temperature is increased above the specified design temperature due to fire or exothermic or runway process reaction, the metal temperature, specially the unwetted portion of the vessel, may reach a level which stress rupture could occur. The use of vapor depressuring is one of method of avoiding such an occurrence." Is it correct? Please advise.

More over, how to calculate blow down and depressuring load? As far as I know the flare header size is determined by taking into account a relieving load, blowdown load, depressuring load and minimum purging rate to keep the gas on the flare burns. Please advise if my understanding is not correct.

any comments will be highly appreciated.

thank you
regards,
bernath

[paulhorth]

Bernath,

To reply to the points you raise:

You misunderstand the action of a PSV. A PSV will never reduce pressure in the vessel. It is closed by a spring, and if the pressure falls below the set point, the valve will close, shutting off the outflow. Only a valve which stays open will depressure the vessel. Suitable devices are a BDV, a bursting disc, or a buckling pin valve.

Blowdown and depressuring do mean the same thing, at least in the sector of the industry that I work in (blowdown of a steam boiler is something different). To calculate the blowdown flow, based on reducing the pressure within a certain time period, you can use a simulation package such as HYSYS, but this does require experience to specify all the parameters correctly. The simulation will take into account the generation of vapour from the boiling liquid with heat input from a fire.

The statement that you quote, about the reason for depresssuring, is exactly correct.

The load in the flare header during a fire needs to be carefully assessed. If you have a blowdown system, and it functions corrrectly, then there will not be a simultaneous relief load because the vessels will not reach the relief set pressures. If you do not have a blowdown system, then the combined load from all the PSVs of equipment in the fire area should be used. This load also represents what would happen if the blowdown system fails to operate. However, typically, the flare load from blowdown of all affected vessels will be greater than the relief load, so this is what will govern the flare header flowrate in a fire.
There may well be greater flaring loads for non-fire cases, for example relief from a high pressure pipeline at the plant inlet, if the plant is blocked in. This flow can be higher than the normal feed flow.

Paul

[bernath]

The load in the flare header during a fire needs to be carefully assessed. If you have a blowdown system, and it functions corrrectly, then there will not be a simultaneous relief load because the vessels will not reach the relief set pressures. If you do not have a blowdown system, then the combined load from all the PSVs of equipment in the fire area should be used. This load also represents what would happen if the blowdown system fails to operate. However, typically, the flare load from blowdown of all affected vessels will be greater than the relief load, so this is what will govern the flare header flowrate in a fire.
There may well be greater flaring loads for non-fire cases, for example relief from a high pressure pipeline at the plant inlet, if the plant is blocked in. This flow can be higher than the normal feed flow.

Paul

Dear Paul

thank you very much, the explanation was very excellent and thorough.

I still have two questions regarding on your statement in the last paragraph.

For PSV, the one that trigger the pop up is when the pressure inside the protected equipment reaches the set pressure of our PSV. I still don't get it of what will trigger the blowdown (BDV) valve to open? What is the exact parameter that triggers the opening of BDV. Is it the temperature that emerges from external fire? or is it anything (I mean anything at all) that brings emergency shutdown status to our system?

If you have a blowdown system, and it functions corrrectly, then there will not be a simultaneous relief load because the vessels will not reach the relief set pressures.

I'm still not clear about this. Blowdown can only happens at fire case (strictly saying, emergency shutdown condition) while PSV can happen anytime when pressure inside the equipment increased to set pressure (i.e. non-fire case). Please advise if I'm wrong.

Sorry if my questions is indeed very basic. I'm still a newcomer to relief and depressuring area

any comments will be highly appreciated

thanks
regards,
bernath

[paulhorth]

Bernath,

Blowdown is an instrumented system operated by the ESD system. The emergency shutdown philosophy developed for the plant will define when a blowdown takes place. It can be triggered manually (by a push button in the control room or locally), and maybe sometimes by other causes including fire detection and gas detection. It is not triggered by high pressure.

There are usually several levels of ESD, with the highest level being the one which is implemented manually in a major disaster when the plant is being evacuated. This level of ESD will usually set off an automatic blowdown. Lower levels will not usually cause an auto blowdown, this is left to the operators to decide. Onshore and offshore practice are different on this point. If there is a minor fire you don't want to have an automatic blowdown but you want to implement blowdown if the fire is judged to be serious. Non-fire cases such as gas detection should shut down the plant, but blowdown should still be an operator decision. A crisis such as gas in the control room might be serious enough to trigger auto blowdown.

All these considerations should be defined in the Shutdown Philosophy, an important component of the design of any plant. Different oil & gas companies have different standards and practices in this area.

In my post below, I mentioned bursting discs and buckling pin valves. These devices ARE triggered by high pressure, and are not used for blowdown because they cannot be opened by the ESD system. I mentioned them as examples of valves which will stay open, in contrast to a PSV.

Paul