4 replies to this topic

[LeoLeo]

Hi all,

 

I want to ask your opinion regarding modifying set pressure based on liquid overfilled scenarios .

Based on ASME, design pressure for pressurized vessels is identified for top of the equipment. In many cases, we consider set pressure equal to design pressure.

 

Now, imagine a vertical vessel (height of 10 m and design pressure of 10 barg) and PSV is located at top of the vessel. For liquid filled scenario (with water), we consider relief pressure of 11 barg (design pressure plus 10% over pressure). This means the pressure at bottom of equipment will be 12 barg (relief pressure plus liquid static head).

 

The set pressure has to be adjusted based on location of PSV and liquid static head. I believe we need to consider liquid static head from outlet nozzle of equipment (on top) to PSV inlet as general rule and not consider liquid static head from bottom of vessel.

 

In some cases in industry we have different design pressure for top and bottom. In these cases, we need to assure we do not exceed design pressure of bottom for overfilled scenario.

 

Do you agree?

 

ARM

[Pilesar]

Pressure vessels are built to consider the design pressure to be at the top of the vessel. Generally, the vessel pressure at the bottom is calculated to include liquid static pressure of a full vessel. Check this in your archive of the pressure vessel calculations if there is any doubt. Water is usually assumed in the calculations unless the contents are known to be denser than water. There are other vessel stressors considered in vessel construction resulting from holding up the mass of the vessel itself, earthquake shear, wind shear, etc. These are all managed when the vessel is fabricated so that the pressure at the top of the vessel is considered to be the reference point for relief calculations. This is much simpler and is conservative as long as the original vessel calcs were accurate.

[ankur2061]

Hi,

 

Look at the basics of over-filling a vessel with liquid.

 

First, you need to consider that a liquid overfill is a credible relief scenario and evaluate it with other overpressure scenarios and decide whether this will be a governing case for vessel overpressure protection. The evaluation includes operator action to stop filling on high level alarm as well as automatic cut-off of filling by closure of automatic valve in the fill line due to high-high level. In most cases, liquid over-fill can be mitigated by the aforementioned steps and does not remain a credible relief scenario.

 

If liquid over-fill is credible and governing for vessel overpressure then the following needs to be considered.

 

1. The source of the overfill:

As an example, let us say that a centrifugal pump is pumping liquid into the vessel and the the vessel overfills without any outlet for the liquid.

 

2. Maximum pressure the vessel top can see due to overfill:

Also for the example considered the fill nozzle is at an elevation above the pump discharge connection and on top of the vessel. In such a case the maximum pressure the vessel top can see is the pump shut-off pressure minus the static head from the pump discharge connection to the fill nozzle.

 

From the above two points you need to check the design pressure of the vessel (which is considered from the vessel top) against the calculated maximum pressure. Based on this comparison you can set the overfill liquid relief device set-pressure. In most cases setting the overfill liquid relief device to the vessel design pressure will suffice.

 

Hope this helps.

 

Regards,

Ankur.

[Jiten_process]

As I understand your case relating to the case i have come across in my project, safety valve is placed at higher platform to have discharge line free draining slopped to your flare header. however, vessel is placed at a lower elevation and you have liquid overfill scenario. 

 

to answer your question, i would assume herewith that you have analyzed your system in totality as suggested Ankur and concluded that liquid overfill scenario is credible overpressure scenario. 

 

Then the answer to your question is "YES" and "NO". This is as per Shell DEP 80.36.00.30. below is the extract from the same

 

Clause 5.3.2 page-17 " Liquid relief - For vessels in liquid full service, relief device set pressures shall be adjusted to compensate for static head between the relief device and the protected equipment. For vessels that normally have a vapour space and are subjected to a liquid relief scenario,the relief device set pressure shall not be lowered to compensate for the static head if that result in exceeding the allowable accumulation while relieving.

 

YES - If vessel is in normal operation complete liquid fill, you need to account for static head elevation while setting PSV set pressure and ensure that when PSV lift, the pressure in the vessel shall not be higher than MAWP. Take a note of not taking credit of overpressure above MAWP as normally is the argument. E.g. liquid coalscers, pre-filter vessels in oil stabilizer trains. 

 

NO - If vessel is operated with vapor space during normal operation, do not take into account the effect of static head. E.g. three phase / two phase seperators

 

Let me also re-phrase some of clause in DEP 80.45.10.11

 

Clause 2.4.2 - "The design pressure (or MAWP) for the pressure system SHALL [PS] be based on the

lowest design pressure (or MAWP) of any system component. The effects of static head

shall be taken into account."

[gegio1960]

My experience:

- Deduct the positive static head between the PSV elevation and the top of vessel from the vessel design pressure (Pdes) to obtain the PSV set pressure.

- Consider the greater between SG=1 and the cold SG of the process liquid to calculate the static differential pressure

So that, if you have 10 m between the top of the vessel and the PSV elevation: Pset = Pdes - 0.1 (P in bar or kg/cm2).

Simple and effective.

Moreover, the accumulation shall not be considered in the Pset definition.

Good luck!