3 replies to this topic

[ing80]

Dear All,

I'm a junior process engineer and it is my first time that i write on this forum (but i often see it).

i'm studying on process for little plant. The aid of plant is to stabilize the oil coming from well and is mainly formed by three gas/liquid separators working a different pressure. Between second and third separator and after third separator are necessary centrifugal pumps.

The flow rate coming from wells is very variable during the life plant, so the pumps have to handle in the first period about 1,100 Oil bbl/day and in the second period 4,400 Oil bbl/day.

What is the best way to design those pumps?

a) To use n pumps + 1 spare. The first period a part of pumps is off and moreover when the plant is at design capacity all are run (excluded the spare pump).

To use Variable Speed Driven Pump

Can VSD work at 20%-25% of Nominal Flow Rate.

Thanks at everyone.

Ing80

[R NESAMANI]

It is good to use VSD driven pump. As this will save you more energy in your case.
But i dont think that the pump can be run less than 50% of flow.

[ankur2061]

ing80,

VSD's are a common method to control flow rate from centrifugal pumps but the motor for the pump has to be designed for variable speed. In oil and gas applications VSDs are economically beneficial when you are handling very large flow rates otherwise the life cyclce cost for a VSD may not be justified for small pumping rates. By life cycle cost I mean both the Capital Expenditure (CAPEX), Operating Expenditure (OPEX) and ultimately the payback for the entire VSD system.

As to your other question about turn-down in the capacity, VSDs can provide excellent turn-downs of upto 4:1 or in other words 25% of the rated pump capacity.

Regards,
Ankur.

[kkala]

To the useful posts already sent, I woud like to add points as below.
A. Local refineries do not to use VSDs in general. Asking why, the answer was "because of the hazardous areas". Search on the web indicates following.
A1. A VSD motor releases more heat than a normal motor, so it requires special provisions to be suitable in hazardous areas. See http://www.reliance....notes/d7736.pdf, VFD (variable frequency drive) is another name for VSD.
A2. For motors operating in hazardous areas, their VSDs are located far from them, in "clean areas", e.g. in control rooms (or with motor control centers, I assume). Recently it has been possible to install VSDs in hazardous areas. See above web reference, as well as http://www.todaysfac...ouse-hinds.html.
A3. So there are VSD motors suitable for hazardous areas, their extra cost impact has to be assessed.
B1. In an alumina project in the past (1993) a lot of VFD drives were purchased, with max rotational range about 2 : 1. Modern VFDs are smaller in size and of higher range, apparently satisfying requirements by ing80. See http://www.practical...m-range-222075/, even though they concern a lathe.
Note: Pump affinity laws are not precise for such a broad range. See http://www.eng-tips....=198102&page=23. Better to specify low and high requirements (flow, head, etc) for the VSD pump and let vendor define RPM range (if this option is selected).
B2. What if the installed VSD pump needs maintenance? You may need another pump as standby.
C. Probably a way out is to clarify the two options, constant speed pumps or VSD pump(s), estimate their capital and operating cost, tabulate their data and have the decision.
Note: Constant speed pumps might be a simpler, yet more expensive solution (personal opinion).

Note on 4 Oct 2011: Again seeing why local refineries do not install VFDs , I understood they are reluctant due to lack of experience, mainly on maintenance. Have VFDs been adopted in other refineries?
This may seem too conservative, however I remember 3 years of fruitless effort to calibrate H2SO4 flow into a phosphoric acid plant, when a "modern" control system was installed (1977-80). The old "primitive" system operated satisfactorily, smoothed by long wide application.

Edited by kkala, 04 October 2011 - 09:46 AM.