5 replies to this topic

[process705]

Hi every One:
Simple Question, why do vendor use larger suction line and
smaller discharge line in Centrifugal pumps?
How does it effect process?
why they don't apply the same philosophy in PD pumps?
Thanks allot

[Art Montemayor]

Process:

It is vendors who design or select the nozzle sizes on pumps. It is the designer/fabricator who does that.

You would have to ask a fabricator that question in order to get an authoritative answer. I can only furnish my opinion. My opinion has always been that the fabricator fixes the discharge nozzles size first, based on acceptable pressure drops and velocity. He then fixes the suction nozzle size such that a liberal Net Positive Suction Head is achieved. The results ofter turn out to be that the discharge nozzle is 1 size smaller than the suction nozzle.

I have seen the same philosopy employed in gear pumps - and they are positive displacement types.

[ARAZA]

Art,

The size of the inlet and the discharge ports depends on the capacity, higher the capacity, more pressure drop and more chances to boil the liquid if the pressure drop is more. Hence to aid in higher NPSH, most of the time the suction nozzle is one or even two sizes more than the discharge nozzle.

I've seen suction and discharge ports of the same size in most of the postive displacement pumps, once again I'm talking of the smaller capacity pumps since I've been working in pilot plant industry for quite some time. There will not be same for higher capacity pumps.

Centrigufal pumps are utilized when there is a demand for higher flow at moderate pressures. Hence all the time, we find difference in sizes between suction and discharge nozzles sizes.

ARAZA

[Muhammad Adnan]

Hi every One:
Simple Question, why do vendor use larger suction line and
smaller discharge line in Centrifugal pumps?
How does it effect process?
why they don't apply the same philosophy in PD pumps?
Thanks allot

Dear Process;

In my opinion, centrifugal pump are designed for a constant head and positive displacement for constant flow.

In industry, we are normally maintaining system pressure at some certain value with some variation limits. So, with variation in pressure, temperature and other parameters, flow across the centrifugal pumps are automatically changing to get the required pressure [Remember P = Density*gravity force*Head] so, required NPSH [Net Positive Suction Head] is also continuously changing. To mitigate this problem, suction nozzle size is normally larger than discharge as at discharge you have excess pressure drop margin as compare to suction.

In PD, no such problem is observed, as they take a fixed volume at suction and then work on it to increase its pressure. So, no significant change in suction flow observes. Therefore, no problem of change in required NPSH observe. Hence no requirement to increase suction nozzle size.

I hope it may have resolved your problem.

Take care and and feel free to ask any question.


Best regards,

Muhammdad Adnan Hanif

Process Engineer

adnan_hanif@ffc.com.pk

[Qalander (Chem)]

I should say, it is the system/process designer to set such condition for a near match from vendors.
Actually the services are clearly defined as requiremets on initial query data sheets.
Now principally the centrifugal pumps are just (Pressure enhancing machine) in a way for the desired flow-rate load range.

Now the driving forces available for that flow at the suction are lesser(head) thus we require pipeline dia has to be more.
At the discharge side Driving forces are much more enhanced to obtain(ensure) the dischharge side same flow.
Hope this clarifies the situation somewhat I believe!

[Qalander (Chem)]

Sorry I missed "that's why discharge pipeline dia is lesser" and flow is converged to an upward pressure condition changing increased Kinetic energy into (higher)head.