6 replies to this topic

[Skyline]

Dear all,

I would like to ask a question about pump selection. I work in Hydrocracking plant, where we commonly use turbine pump when running in normal condition, and backed up by motor pump as a spare pump. 

But, in some equipment, we use turbine pump as primary pump and backed up by turbine pump as a spare pump. This pump transfer the bottom of low pressure separator (liquid hydrocarbon) to debutanizer.

I also found a pump configuration where both the primary and spare pump are turbine pump. This pump is diesel pump around (hot wash). 

Do you know what is the reason behind these configurations? Any idea? Thank you very much before.




Best regards,




Skyline

[Zauberberg]

Normally, choosing two different types of pump drivers is mainly done in those cases when you want an extra sort of redundancy, i.e. when power failure will not leave you without both pumps and move the plant operation into hazardous shutdown. We had the same configuration in Crude Distillation Unit where both the Crude charge pump (unit feed) and Atmospheric Residue pump (main column bottoms) were driven by steam turbines, and having fixed speed motors on spare pumps.

Additionally, turbine driver provides you with an option to save energy since there is no requirement for a control valve downstream of the pump in majority of cases. For pumps that are large energy consumers this can be a very significant factor.

[Art Montemayor]

Sky:

I believe any response(s) you generate are only going to reveal confusion or misconceptions.

You furnish what is a generalized, un-specific set of equipment definitions or descriptions. Allow me to illustrate the confusion:

turbine pump – this description is subject to interprettion. A turbine regenerative centrifugal pump is one type. A vertical, multi-staged centrifugal “turbine” design is another. What, specifically, do you mean?

motor pump – most pumps (especially centrifugal types) are driven by electric induction motors. What is so special about this one? Please be specific about the pump type. The driver makes little or no difference in the pump’s pumping performance.

diesel pump – is this a TYPE of pump? I doubt it. Perhaps it means a pump (yet to be defined) that pumps Diesel. Practically any pump is capable of pumping Diesel. What is so peculiar or special about this pump?

Please be SPECIFC about the pumping application. Tell us essentially all that can be found in a Pump Data Sheet. In fact, why don’t you simply supply us with the correct and accurate Pump Data Sheet for this application – together with a schematic sketch of the application. That would be much more simpler and accurate to enable Forum members to comment on.

[chemsac2]

I have seen turbine + motor arrangement for overhead reflux pump and for residue pump in main fractionator of hydrocracker plant.

Overhead reflux pump is provided with that kind of arrangement to ensure atleast some cooling in event of power failure. Section 5.6 in API 2007 aasks designers to arrange overhead reflux and pumparound reflux such that simultaneous failure of two does not occur.

Residue pump is generally provided with steam turbine + motor configuration and a connection from residue pump to fractionator feed heater to avoid fired heater running dry in event of feed trip on power failure.

However, I have doubts on effectiveness of this method. To ensure equal ageing of both pumps, motor and steam turbine pumps need to be alternately operated. Now if motor driven pump is running and power failure occurs, turbine pump may not auto-start as it requires certain time to be started.

Regards,

Sachin

[Nikhiln]

I will assume you are talking about pump-DRIVER selection and not pump selection, as you can see the confusion you have created in Art's mind (and probably, in many others too, which is justified, since you used the term turbine-pump). If your query is not about drivers, please ignore this reply.

Selection between turbine-drive & motor-drive is a function of mainly 2 criteria:
1) Utility costs
2) Criticality of the pump operation

Cost point of view: The normally running driver should ideally be that of lower utility cost. In some cases, cost of power could be less than cost of steam based on the plant's location. Thus in those locations, you would want the main pump & the standby pump to be motor-driven as well.
Criticality point of view: Motor-driven pumps are the usual preferred standby pump, where the operation is critical (like, reactor coolant loops) where flow cannot be allowed to stop even for a few seconds. They can immediately cut-in on autostart. However if your steam cost is low AND the operation is not so critical, so as to allow a few minutes stoppage of flow, you can have your standby pump also to be turbine-driven. In such cases, the standby turbine will usually be on hot standby, where the exhaust steam valve will be open and a small turbine casing valve as well, to keep the turbine warm.
it might still take a couple of minutes to reach full operational speed, though, since the steam governor mechanism will not just go flat open immediately.

So driver selection is a result of considering these two main criteria, along with others like, control aspects (in revamp cases, the overall steam balance of the plant might have a say too).

Cheers!

Edited by Nikhiln, 10 August 2010 - 01:10 AM.

[Skyline]

Dear all,

I'm really sorry for the confusion i've made. Let me explain what i meant: I'm asking about the selection of a driver for a particular pump. I'm really confused about this. I attached a simplified process flow diagram as a reference. Please refer to that drawing.

First case: Centrifugal motor driven pump backed up by turbine driven pump, example: P-05 Pump. This pump transfers liquid hydrocarbon from low pressure separator to debutanizer.

Second case: Centrifugal turbine driven pump backed up by turbine driven pump, example:P-17 Pump. This pump acts as a reflux (hot wash) of diesel fraction in fractionator column.

Do you have any idea about what might be the reason behind these configuration? What is the influence of the driver characteristic in this selection?

Again, sorry for the confusion earlier. And thank you for your replies (also for Art for correcting me).



Best regards,



Skyline

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[Zauberberg]

Skyline,

I think you have already got the answers on your question - read the posts from Nikhiln, Sachin, and myself.

Best regards,