30 replies to this topic

[M.Orojlu]

Dear All,

In many projects the piping class (Rating) of pump suction (especially from pump suction block valve to suction) is the same as pump discharge line.

1- What is the reason?
2- Does any criteria exist for this matter or we should do the same for all systems?

Regards,

Mehdi Orojlu

[djack77494]

Medhi,
This is not an arbitrary or illogical practice. If this and similar concepts are fully thought through, the underlying reasoning can often be uncovered. When multiple pumps can operate in parallel, it is possible for someone to open the discharge isolation valve for the pump on standby in preparation to switching to that pump. The suction isolation valve may be closed at that time. Thus the pressure at the discharge of the operating pump can exist back to the standby pump's isolation valve. Therefore, the piping back to and including the suction isolation valve should be rated for discharge pressure. Make sense now?

[JoeWong]

Besides parallel pump as explained by Doug, for single pump installation, the discharge design pressure may need to be extended to the pump suction valve or even upto last isolation valve of the pump suction vessel. Reason being overpressure from other source on the pump discharge line could be extended upto to the last valve during shutdown period...

[fallah]

I think, considering possibility of operating pump (specially high head) trip,because all parts of suction line subjected to pressure shock of created water hammer,suction piping rating (even before isolation valve) should be equal to that of discharge.

Dear All,

What was stated for the suction including the valves should have the same rating as the discharge is valid 100% for the centrifugal pumps but for the reciprocating pumps I do not think so since there is valves within the pump to prevent the back flow. May be you will say that it is check valve only but there is two check valves , the other one is on the discharge line. Please provide your argument if you think that still the rating of the suction should be the same as the discharge.

Regards,

[fallah]

If the discharge of PRV (usually located at reciprocating pump outlet) is to be connected to the pump suction line, the rating of the suction line should be the same as the discharge.

[Qalander (Chem)]

Dear fallah,
I would like to differ in view of the
actual suction pipeline geometry and
its inter-connection with the pumped-out system storage(s)
Since if we have a nearby connected/circuit ensured open bulk storage of low (near-atmospheric pressure); then most usually 150 psig rating suction pipeline flanges/valves and fittings are observed to be used.
Other learned forum colleagues will put the record straight if this is narrated in-correctly!
Best regards
Qalander

[JoeWong]

Dear All,

What was stated for the suction including the valves should have the same rating as the discharge is valid 100% for the centrifugal pumps but for the reciprocating pumps I do not think so since there is valves within the pump to prevent the back flow. May be you will say that it is check valve only but there is two check valves , the other one is on the discharge line. Please provide your argument if you think that still the rating of the suction should be the same as the discharge.

Regards,

Argument could be check valves leak or passing. Check valves do not provide sufficient Safety Integrity Level to safeguard the suction piping...

[djack77494]

What Mr. Wong says is true even for two check valves in series, which is what typical PD pumps have. If you have two PD pumps configured in parallel, one running and one spare, then the suction piping of the spare pump could see the discharge pressure of the running pump. All it would take is for the spare's discharge isolation valve to be open, suction valve closed, and back-leakage through the pump.

Djack,

I agree with you but if there is two pumps in parallel and the suction/dicharge valves of each pump are locked open how the suction will see the same pressure as the discharge, please explain?

Regards,

[pacific]

Hi Djack,

Back flow from operating pump to spare pump has a little possible
Because in this case besides
discharge valve of the spare pump should be open and it's suction valve should be close ,
check valves must be failed too.

The major reason for similarity of design class in discharge pressure line, and block valve to suction of pump can be categorized as below:

1-what JoeWong said in 3rd post
2-In pump start up stage, operator forgot to open the discharge valve and backflow could occur

Regards,
Pacific

[fallah]

2-In pump start up stage, operator forgot to open the discharge valve and backflow could occur

If you are discussing about PD pump,when discharge valve is closed,its upstream PRV will be activated and backflow will not occured.

[djack77494]

Djack,
I agree with you but if there is two pumps in parallel and the suction/dicharge valves of each pump are locked open how the suction will see the same pressure as the discharge, please explain?

We must imagine that through inadvertent operation of the valving, the discharge valve of the spare pump is opened before the suction valve. For the time period between the two events (which may be seconds or years) the spare pump's suction piping could be subjected to the operating pump's discharge pressure. It is very uncommon, and I would not recommend the practice of locking open the valving around pumps like this.

[JoeWong]

Djack,

I agree with you but if there is two pumps in parallel and the suction/dicharge valves of each pump are locked open how the suction will see the same pressure as the discharge, please explain?

Regards,

During pump maintenance, your operator have to ensure the following are in sequence :

Step 1 : Close discharge valve
Step 2 : Close suction valve (remove lock before)
Step 3 : Open drain valve

First you have the probability of discharge valve and check valve passing after step 2 but before step 3 is implemented. The probability is rather low. However, a safety integrity studies will be required to prove this event is within the acceptable level.

Second, you have the probability of operator implement step 2 before step 1. This is something control and manage by operator. How to avoid operator does not make mistake ? Some operating company imposed stringent maintenance procedure. Nevertheless, the risk is still present. Some measure may be required. Some engineer may consider implement interlock among these three valves.

In view of above requirement, you may think twice if you should fully rated the suction piping to discharge piping.

[fallah]

The spare pump foreseen to start automatically with open valves(suction and discharge),is a typical example of the case in which the suction line may be subjected to discharge pressure due to check valve passing.

[djack77494]

The spare pump foreseen to start automatically with open valves(suction and discharge),is a typical example of the case in which the suction line may be subjected to discharge pressure due to check valve passing.

Rating a (usually) short section of suction piping for higher pressure is a small price to pay for the added flexibility and safety that will result.

[Qalander (Chem)]

The spare pump foreseen to start automatically with open valves(suction and discharge),is a typical example of the case in which the suction line may be subjected to discharge pressure due to check valve passing.

Rating a (usually) short section of suction piping for higher pressure is a small price to pay for the added flexibility and safety that will result.

Dear All/ M.Orojlu(Topic Opener)
May I just summarize? As discussed above; it is not uncommon to have small suction pipeline segment same pressure rating as pump discharge at a bare minimum cost.

However the possibility of having the same pressures at this segment may be observed as indicated above last 3~4posts by M/S Joe, fallah and Doug.

I still have some questions to ask these three learned colleagues that when a suction loop is flowing in from a storage towards one of these pumps (see Attached rough sketch),

Is there any true restriction on excess flow recycle-back towards the suction storage?
If none then;why and how the flow of a parallel pump should not go back to the same suction storage?

Kindly explain with example as the suction line-up does not have any check valve(s) and only Isolating Block (gate) valve are kept wide opened for pump-out.
Regards
Qalander

[djack77494]

Qalander,

I have some questions and issues with this posting. Foremost, I looked over your sketch and did not like what I saw. Remember that we are discussing positive displacement pumps; you depict the pumps using the normal symbol for centrifugal pumps. Your PSV's were not correctly located; they must be located INSIDE the discharge isolation valves to protect the discharge side of the pumps. I would not show a check or non-return valve at the discharge of the pumps; they are not required in light of the non-return nature of the pumps themselves. Your PSV's should be shown rotated 180o so that their discharge connections are to the left (which I know you intended as the discharge line does go to the left). Finally, I would prefer that the PSV's discharge back to the suction source, rather than just upstream of the pump, though this is a preference and not necessarily a requirement. (Sorry about the many comments, but I can be very nit-picky.)

I do not really understand your questions. What do you mean when you say, "excess flow recycle-back towards the suction storage"? I think you mean the fluid flowing through the PSV. If so, then when you inquire about a "true restriction", this would be the orifice inside the PSV. Again, I do not understand your question about the flow from the parallel pump. The parallel pump as shown is isolated on its suction and discharge sides and is not running. There will be no flow. There is no reason to put a check valve in either pump's suction line since there is no concern with backflow (remember the PD pump has two internal chack valves to prevent backflow) so I don't understand why you'd ask about a suction line check valve. Frankly, I have never seen a suction line check valve for any pump and I never expect to. Lastly, I believe the isolation valve you say would be wide open for pump out is the same as the one you show as closed in your sketch. Please be more specific, perhaps even labelling your valves "A", "B", "C", etc to better explain your concerns.
Regards,
Doug

[Qalander (Chem)]

Thanks Doug for your kind reply; however, explicitly I'll come back later.

Presently only I refer to topic opening text, that does not define positive displacement pumps in any way. That's the reason I preferred using centrifugal pump's reference. Kindly check/confirm this; do correct me if I am wrong.
Best regards
Qalander

[djack77494]

Qalander,
Please excuse me-you are correct in reminding us that the early postings to this topic seemed to be more focused on non-specified or centrifugal pumps. Since so many of the postings were refering to reciprocating pumps, I lost sight of the topic's origins.

Despite that, I believe that most of my comments remain valid. So, when you have the opportunity, please revisit.
Thanks,
Doug

[Qalander (Chem)]

Qalander,
Please excuse me-you are correct in reminding us that the early postings to this topic seemed to be more focused on non-specified or centrifugal pumps. Since so many of the postings were refering to reciprocating pumps, I lost sight of the topic's origins.

Despite that, I believe that most of my comments remain valid. So, when you have the opportunity, please revisit.
Thanks,
Doug

Dear Doug,
Revisiting was and is my committment and most of your pointed out items are almost 100% applicable w.r.t. my post 2nd last post; but I could not muster out time presently.

[M.Orojlu]

Dear all/Qalander,
Thanks for your writing. Excuse me for replying to your quotation with delay.
For recycle line (Min Flow line) from haeder or after chaeck valve should be connected to Storage tank with related restriction orifice or ctrl vlv not to suction line.(see attached your modified sketch).

Also I faced with some high pressure lines with two check/stop valves on discharge lines that suction lines pipe class (after suction stop valve) are the same as discharge lines.

Regards,
Mehdi

Attached Files

•  Rough_Sketch_Showing_two_parallel_pumps.xls   44KB   200 downloads

[JoeWong]

Is there any true restriction on excess flow recycle-back towards the suction storage?
If none then;why and how the flow of a parallel pump should not go back to the same suction storage?

Don't quite understand your questions.

Are these question related to the original intention of Pump suction rating ?

[demank]

Is there any true restriction on excess flow recycle-back towards the suction storage?
If none then;why and how the flow of a parallel pump should not go back to the same suction storage?

Don't quite understand your questions.

Are these question related to the original intention of Pump suction rating ?

maybe the above question is recycling back is what for.,
i think recycling back the pump outlet flow is to maintain the storage tank level of inlet pump. cause if the level of the tank is lowered, it will decrease the NPSH.

[antator]

Reading all the posts above I was hoping someone could confirm where you have a centrifugal pump with a minimum recycle the recycle flow will be throttled back to the suction line/vessel pressure, correct? Also, can anyone provide any possible examples of what might cause back flow, and thus the need to design the suction line for the discharge pressure. Thanks.