hi all,

I have to design a psv on the discharge line of a centifugal pump. The pump transfers fluid from a feed truck to process tank. The scenario considered is as follows.
The pump shut-off head is 6 barg. The valve at the inlet to the storage tank is blocked in resulting in pump developing shut-off head. Also, The feed tank has nitrogen blanketting at 6 barg which is generally purged before running the pump. The operator runs the pump without purging the nitrogen. The total discharge head is now 6 +6= 12 barg whereas the pipe MAWP is 10 barg. How to determine the sizing flowrate for this PSV? Iam new to this field and any help will be highly appreciated.

How do you arrive at the conclusion that "the total discharge head is now 6 +6= 12 barg"?

The total discharge head on the valve at the liquid inlet (pump's discharge) to the storage tank will be the 6 barg of Nitrogen pressure + the static head of liquid in the tank above the same valve (a height you haven't furnished). If your pump has a dead head (shut-off) pressure of 6 barg, then it will develop that – and no more than that. Pressure is not a vector quantity; you can't add the pump's developed head to the head in the tank. The pump will be "dead headed" against any head bigger than 6 barg. This means it is not pumping anything. Additionally, if you don't have a check valve in the discharge, your pump will spin backward

From what basic data you have given, there is no indication that you need a PSV on the pump's discharge. If your operator starts the pump without venting the 6 barg of nitrogen blanket on the tank, you run the risk of overheating the pump due to the dead headed situation (zero net flow). Depending on the type of pump, you may damage the seals first and other parts later. But you would never achieve the 10 barg. After mistreating the pump like that, you may not even achieve the original 6 barg.

If the maximum pressure on the suction side (feed tank) is 6 barg and the pump has a shut-off pressure of 6 bar one would expect a total shut-off of 12 barg (without static height).

One could easily determine where on the curve the pump will theoretically operate, based on set pressure, suction pressure and the pump curve.

CMA010:

You are toying with theoretical "guessing". The poster stated what appears to be a practical and everyday scenario, but he fails to give us all the basic data.

Where would you obtain a truck with a liquid at 6 bargs? Most tank trucks that I have unloaded are at atmospheric pressure - or close to it. I don't believe that is a common nor practical situation. This is what happens when posters don't give specific details and information and leave us to waste time and effort guessing at what they are talking about or what they are dreaming about.

Yes, it could be that the fluid in question is an LPG -- but that is stretching it. LPG are in the range of 12-15 bargs. In fact, if it were true that the suction is at 6 bargs, then it also would be blanketed by nitrogen at that pressure. And if that were so, then what would be the common sense thing to do would be to equalize both the truck and the tank.

My point still is that we haven't been given sufficient information to help in answering the question or problem presented. We are engineers, not fortune tellers. We don't have magical crystal balls that tells us all the basic information that the poster is too lazy to furnish so that we can do his thinking for him/her. We need all the basic data. Otherwise we will all be guessing - like you are guessing that the suction of the pump is at 6 bargs. You don't know that; no one has even hinted at that. You are simply guessing. If all it takes to respond to a query is to guess at an answer, then why have an engineering Forum? The Original Poster (OP) can simply guess the answer him/herself and not waste time writing in to Forums.

sgayat,
There are many out there willing to assist you... Nevertheless your post is lack of information and has lead to unwanted discussion.

PROCESS
Truck ==> Centrifugal pump ==> Tank

Please advise the following :
Fluid = ??
Fluid Vapor pressure = ?? barg
Fluid density @ min & max operating temperature = ?? kg/m3
Operating temperature = ?? (min and max) degC
Operating pressure = ?? (min & max) barg
Is your truck blanketed with N2 ? What pressure ? 6 barg ?
Suction static head = Truck maximum liquid level - pump centre line = ?? meter




Shut-off head is in meter, feet, etc.
Please read from the pump curve the head at zero flow advise.

If you don't have above information, please advise the pump normal head.



Why purge since it already blanketted with N2 ?


This contradict with early statement. Confusing. Please clarify.



Can't just rely on normal suction pressure. There are at least two scenarios you have to consider.

Normal the MAWP is limited by the receiving tank instead of pipe. Please advise the design pressure of the process tank.




If you have the pump curve, please submit. The rate can be read from the curve.


Once you have provided above information, we may helps.

Thankyou for your response. Please see the details below. Iam outsourced so this is all the information i have from the plant.

1. pump details: Material being pumped: isopentane
Pump inlet pressure max= 87 psig ( 6 barg)
Pump inlet temperature = 20 deg C
Dead head pressure = 210 ft of water ( 6.27 barg)
Flow rate at zero head = 97.5 GPM

I was provided with intermediate flowrates and heads based on which i have ploted the Pump curve. Iam enclosing the same as attachment.

When the truck/ wagon arrives there is nitrogen atmosphere at max 6 barg above the liquid. Before starting the pump, nitrogen overpressure in the wagon has to be evacuvated by the operator. If this step is not done, the product will arrive at the inlet of the pump at 6 barg and the pump can add 6 barg more. The scenario is blocked discharge ( at the inlet to the tank. Hence, tank MAWP is immaterial) The piping MAWP is 10 barg. So i come back to the same question. Will the line not be overpressurised ? How to size PSV on this line?

Art,

The OP mentioned in his original post the feed tank to be at 6 barg by blanketing, that doesn't seem like guessing to me. It's just a matter of interpretation. But you're correct, the "quality" of the answer is only as good as the information provided. And as good as this information is interpreted.

It is often frustrating to attempt to assist a poster who has not provided adequate information or has presented material that can be interpreted in several ways. Throughout these forums, you will see numerous examples of correct answers to the wrong questions. You'll also see requests (sometimes many) for clarifications and corrections. You'll see partial answers to incomplete queries, frequently in extraordinarily poor English. It would behoove all the users of these forums to try to accurately, completely, and concisely state their problems (which should match the title). I can assure all that they stand an excellent chance of receiving a high quality response if they will just do this.

Responders and questioners as well need to understand that the forum users come from a variety of backgrounds. There are multiple systems of units in use. Command of the languages of the forum, English and Spanish, varies widely. And finally, the experience of the users varies considerably, and covers an enormous spectrum of industries. Despite this, we can all gain from these forums by trying to help each other. Responders of course help the questioners with answers, but questioners can help responders with effective statements of their own problems. Interested readers also benefit.

Sorry about the rant-now to the problem at hand. After the clarifications, it seems as if the pump discharge piping could indeed be subjected to roughly 12barg if 1) The pump is at/near shut-off, and (simultaneously) 2) The suction tank (truck/wagon) blanket pressure is at its maximum of 6barg. As the engineer in this situation, you must decide if this scenario is feasible. For example, acceptable procedures may be in place that would prevent this unusual combination of events from ever occuring. If it is feasible, then you must either: 1) Design for this worst case scenario, or 2) Ensure that your conditions will never exceed design. Plan 1 is easily achieved by upgrading the piping system to withstand 12barg. Plan 2 could be achieved with a PSV in the discharge piping that would ensure pressure <= 12barg, or by instrumenting the system to avoid the hazardous situation. (For example, reliable instrumentation could be interlocked to the pump such that it would shut down if the truck pressure exceeded 4barg OR if the pump's flowrate was such that the developed head + 6barg would exceed 10barg.) I cannot think of a Plan 3.

Also, you may want to double check your "facts". I notice you claim your piping MAWP(?This is a vessel term?) is 10barg or roughly 150psig. This alone is suspicious because many piping systems are refered to as being "nominal" 150#. In fact, a 150# RF flange, typically the weakest component of a well designed piping system, can withstand 285psig/nearly 20barg at up to 100F/38C; it can withstand >= 150psig at up to T = 550F/290C. So you may not have the problem you think you have.
Good luck,
Doug

Why do you calculate your shut-off pressure based on water?

The density of isopentane at 20 oC is 622 kg/m3. This gives a shut-off pressure of 3.9 bar, or 9.9 barg including 6 barg suction pressure (without static head). Depending on your worst case scenario (medium, minimum temperature and maximum suction pressure) you will not reach pressures above 10 barg.

I Really Appreciate this Splendid accurate reply from depth of my heart as I have learnt that better phrasing while asking is essential to attract early accurate replies. As I myself made a mistake in my posted question.
Best Regards
Qalander

Qalander, you are quite welcome. Always glad to be of assistance.