Operating Fixed Speed and Variable Speed pumps in Parallel

[Sawsan311]

Dear All,

Operating pumps in parallel is a process necessity, selection between variable and fixed speed pump depends on the process rangeability (turndown) as well as energy lifecycle analysis. However, in some revamp situations for increasing capacity, fixed speed may be connected in parallel with multiple VSD pumps instead of a VSD pump specially when there is a limitation in accommodating the foot print of VSD auxiliaries such as chiller unit,

From process control perspectives, the control of the VSD can be under downstream backpressure absorbing the variation in the downstream network, fixed speed on the other hand is controlled via a discharge FCV maintaining the pump close to its best efficiency point.

My question is, do you foresee any impracticality of operating fixed speed pumps with VSD pumps in parallel under common suction and discharge. In my opinion, a proper start-up philosophy is required to ensure a smooth transition between the starting of the VSD and fixed speed pump. In most common cases, starting the VSD pump followed by fixed speed pump would contribute in minimizing the load on the motor starting current of the fixed speed pump and initially establishing the pressure in the network prior to starting the fixed speed pump.

Appreciate your views on the subject, some people express concern of operating fixed and VSD pump together stating that VSD pump would have to obey the conditions dictated by fixed speed pump and hence deviating from the objective of VSD control. In my opinion, as long as fixed speed pump is equipped with a discharge control mechanism via throttling at BEP, there should be no concern in operating a VSD pump with it.

Regards,

 

[bimr]

The answer will depend on the size of the pump, the headloss of the piping system, and the pump curve which are not provided in your post.

 

[TiCl4]

Operating a VFD pump in parallel with a fixed-speed pump with serve to reduce the range of operation of the VFD pump. Due to the increased system pressure from the existing flow, the VFD pump will only be able to turn down so much before the pump head is reduced enough to equal the header pressure - i.e. 0 flow.

As bimr state, this operation is dependent on system design, which you need to provide if you want more specific answers.

You also will seem to have the headache of having two parallel control systems - the control valve for the fixed-speed pump and the flow controller for the VFD pump, both of them affecting the system flow/pressure. Actually getting stable flow control may be an issue with competing controllers like that.

Also, what footprint issues with a VFD are you thinking of? Sure, it needs an independently-mounted drive instead of a motor starter inside a motor control center, but it's really not that much space. Unless, however, you are talking about larger pump. Drives for VFD control can start getting pretty big at 100+ HP.

 

[LittleInch]

This sort of situation is good for when you need to maintain a pretty fixed discharge pressure (+/- 5%), but with variable flow.

This is so system specific that you can't really make generalisations, but I would avoid the two controllers and just use the VFD to maintain a similar discharge pressure as the fixed speed unit has at its BEP.

1 fixed and 1 VFD is very feasible.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Keep the fixed speed drives on base load and the VSD drives on swing load; there shouldnt be control interactions between the two sets if control loops are set up correctly.

 

[Sawsan311]

Thank you all for your views.

With regards to the VFD foot print complexities, the auxiliaries associated with motor chiller can't be neglected specially for pumps over 2.4 MW. By all means, employing a VFD while knowing that there would be recycle operation at reduced pump speed would really be not techno-economically justfied.

Additionally, there are other aspects for the control of fixed speed with a discharge flow control valve set such that the operating point is within 80-110% of the best efficiency point while the other VFD controlled pumps absorb the variation in the downstream system based on system backpressure. I think with this setup the coupling interactions between the fixed speed and VSD pumps would be minimized.

Additionally, from start-up perspectives, the fixed speed pump need to be started against an optimum pressure build up on the discharge which can be fulfilled by the start with the VSD pumps such that fixed speed will not have to experience large throttling against its discharge FCV and hence energy loss. However, starting the fixed speed without soft start feature need to be taken into account when designing the electrical load of the system unlike the case of VSD pumps which by default possess the feature of soft motor start with reduced load on the electrical system.

Appreciate your views on the above.

 

[georgeverghese]

The fixed speed pump would have a min flow recycle control loop? So you can start this pump with discharge FCV temporarily closed and min flow loop active, and install a soft start for this pump at MCC if required.

 

[LittleInch]

Well it's quite difficult here to comment on a vague system description as the importance of one factor over another is impossible to judge.

For start-up, if you're concerned over runaway / overflow of the fixed speed unit, just start the pump against a closed control valve and then slowly open it controlling on flow. Any small energy loss is negligible.

VFDs are not always the best solution as they leak energy continuously and it's worse at low power. Each system is different in terms of system curve(s), operations, turn down, time running at different flowrates so you can't have a one size fits all policy here. Sometimes VFD is the most efficient / lowest cos solution, sometimes it isn't. Just because you're throttling a fixed speed unit doesn't always mean a big energy loss. Pumps use less power with lower flow. so long as your pump is sized properly to match the pressure required for the duty flow then it may use less power than a VFD unit.

If you're hurting for electrical supply on start, then soft start units are a lot cheaper than full VFD.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.