3 VFD pump + 1 steam turbine pump to a common header 1

[skyandsea]

Hi All
Currently our company is working on the project that existing system have three electrical motor pump and one steam driven pump for the chilled water secondary loop. And we're going to purchase 3 nos of VFD for the electrical pump is the cooling load reduced from last few months.

The four pumps are pumping chilled water from the cold water storage tank and are linked to the common header. After the common header and branched into two large distribution loop and two loops ends to the warm water tank.

Now, we're thinking to install pressure transmitters on supply and return header of each of the two distribution loop. And compare the two differential pressure to use the lower value to control the VFD.

However, the for steam turbine pump, the governor is manual but not automatic that it's better to run at fixed speed. Any idea to maintain the steam turbine pump at full (or fixed) speed and using the dp to control the rest of the VSD pumps?

Need some inputs from yours. thanks.

 

[thkurian]

As the intention is to reduce head for the same flow for motor driven pumps by the use of VFD, steam turbine driven pump with constant speed ( high head)will deliver the major share of total flow. So it is possible to operate 4 pumps in parallel. But instrumentation is not clear. Unless some of the users are bypassed, pressure differential will not change. If users are bypassed, differential pressure signal is OK for VFD control. Otherwise temperature measurement of chilled water for VFD control is suggested.

 

[LittleInch]

Sorry, but I can't work out your system from the words. Can you sketch this and post it so that we can see which pumps are where and where the flow is individual and which is common.

Then it will hopefully become clearer. Also some idea of flow rates and pump curves would be good

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

 

[Artisi]

Like LittleInch -not too sure what you are doing yet. But one point to consider is running the electric pumps at reduced speed will lower the discharge head whereas the steam turbine unit at full speed will still be trying to produce a higher head - can see a few problems there.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[skyandsea]

Hi all
I've attached the sketch drawing.

You may find there are actually A-F pumps, A-E are electrical pumps, and F is steam turbine pumps. Currently only A/B/C are running and we are going to install the VFD on A/B/C pumps and together run A/B/C & F (steam turbine pump) in the future to balance the secondary flow and primary flow of the chilled water.

I've only sketched the secondary loop, as there is no direct interconnect link between primary and secondary loop, chilled water from chiller to be stored in cold tank and pumped by secondary pumps to the off takers. And return to warm tank.

All the pump design discharge head is about 5 bar and flow rate at 4202 GPM.

For the temperature control concept, it might not working in our plant as our chilled water not only supply to HVAC but also to process where the temperature demand varies.

We'd like to utilize the as much as steam (by product of trigen) to drive the F pump,but in order to address the differential head problem, governor is able to reduce the steam flow to the turbine to reduce the flow and head of the steam turbine pump.

I'm just wondering if any way that able to fully utilize the steam at full speed, meanwhile also use the VFD to varies and blance the flow.

 

[georgeverghese]

How about split range pressure control on common pump discharge? Am presuming the pumps all have the same Q-h profile and slope around the operating point is not flat.

Say, if your required discharge pressure is 500kpag, we set this PIC at 500kpag. Configure this controller as a reverse acting PIC (increasing dev from SP results in decreasing controller output). Send the 50-100% output signal from the PIC to operate the VFD pumps, and the 0-50% output to the steam turbine drive pump.

That way, the steam turbine pump will be base loaded, and the VFD pumps will take the swings in demand.

VFD pump operates at 100% speed at 100% PIC output, and minimum speed at 50% PIC output.
Steam turbine pump operates at full speed at say >50% PIC output, and min speed at 0% PIC output.

If the lowest chilled water demand in this sec loop is below the full speed delivery of the steam turbine drive pump at 500kpag, then this pump governor will have to be repaired to make it run on variable speed.

Presume you have min flow recycle loops on these pumps which you havent shown for the sake of clarity.

There are further refinements possible, but this would be suggested bare bones process capacity control scheme.

 

[skyandsea]

Thanks georgeverghese for the advise. But unfortunately, the governor at steam turbine pump is not automatic but manual.
While the steam turbine pump was installed and commissioned by a different area group and not sure if can spend for upgrading to automatic control.

 

[georgeverghese]

In that case, a dedicated max pressure recycle PIC will be needed for the fixed speed steam turbine driven pump - hope you get the picture. You could modify the min flow recycle loop for this pump to act on max pressure control also via an auto signal select ( hi / low select depending on how you've setup the min FIC / max PIC).

 

[LittleInch]

skyandsea

Your description and the sketch basically show a set of parallel pumps with in flow terms a common inlet header and a common outlet header. As such the pressure difference between the two will be same for all pumps and therefore your original suggestion would appear to be quite odd and IMHO unworkable.

I cannot understand why, if these four pumps are the same or very similar to each other, you actually want to use a VFD. Given that the fixed speed pump F will give out a certain diff pressure, unless you are on a steep part of the curve, your VFD will need to be a quite a high speed to match the head output. With what I guess are 4 x 25% or even 3 x 33% pumps, you would seem to have enough flexibility with fixed speed units to only have as many pumps as you need.

I have difficulties understanding from the data so far, what happens to the flow - does it stay constant and the temperatures change as the load changes or does the flow vary with the load? If it is flow variable, then your best bet would be to control on flow and if you want the steam driven pump to work at 100%, then throttle the electric pump(s) accordingly.

This would seem to be an occasion where VFDs don't actually work or are any more efficient.

We really only have about 25% of the information needed to give a better answer I'm afraid, but hope this helps.

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

 

[georgeverghese]

Just realised the 0-50% ouput range on this PIC isnt doing anything if you run the steam turbine drive pump on fixed speed. So you could route this signal to a hi/ low signal select that compares this signal with the steam turbine pump min flow FIC output and have the output signal from the selector to modulate the min flow FCV. Of course, we would have to rerange the 0-50% signal from the PIC to a full range signal before routing it to the signal select.

This means the min flow FCV for the steam turbine pump will not only have to deal with min flow but also max flow in the recycle line.

In the longer term, may be a good idea to get the steam turbine governor changed out to enable variable speed set from DCS, so you save money on fuel cost for steam generation.

As Little Inch says, you also have power savings by manually turning on / off one or more of the electric drive pumps in fixed speed application. It may be possible to automate this also if you have some idea about how the pump discharge pressure drops off as chilling demand (i.e coolant flow) increases.

 

[ccfowler]

As indicated by others, much more information is needed for best advice to be possible. With the multiple connections in the piping system and the multiple pumps with differing driver characteristics, this system seems to be an invitation to control and pump operation problems. Based on my experiences with multiple mis-matched pumps serving a common piping system, I would anticipate plenty of trouble with instabilities and repeated episodes of pumps running with zero flow. The introduction of VFD's into this mix only assures more waste of energy and greater control complications.

Skip the VFD's. For estimating purposes, they will only assure a minimum waste of about 5% of the energy consumed by the VFD-powered pumps, and depending on the actual pump and system characteristics, the waste could be dramatically more (I would not be surprised by energy waste being increased by 20% or even much more due to the VFD's) due to shifting operation into less favorable regions of the pumps' performance curves.

Put some of the savings from eliminating the needless VFD-forced initial and continuing costs into more useful controls to monitor individual pump flows and optimization of which (or how many) pumps to run to serve varying process needs.



Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[thkurian]

As the performance curves are different for fixed speed steam turbine driven pump and VFD / Motor driven pumps, the capacity sharing will not be equal. As all the pumps were working in parallel before, one will assume the drivers are rated for end of curve power. From the system caharacteristics for the combined flow, find out the lowest head required at the required combined flow. Steam turbine driven pump will operate at this head at it's corresponding flow from the pump curve. Rest of the flow will be shared equally by VFD/Motor driven pumps.

Regarding controls, if flow throgh the Process and Utility users are not controlled, there will not be any pressure differential. In which case, put control valves at inlet lines to Process and Utility which are controlled by temperature at Process and Utility lines outlet. The operation of these valves will cause change in flow in motor driven pump header. This flow signal can be fed to VFD for motor speed control.

Please explain more on Process and Utility distribution and their controls if any.