How to start backup BFW pump without deadheading?

[KernOily]

Good afternoon guys. A (semi) dumb question for you from a pumping system newbie.

I have a large steam generation system consisting of three 900 hp split-case 3600 rpm synchronous speed pumps feeding ten steam generators. They are on auto-transformer start because the power system won't take A-T-L starts. Minimum disch flow for these pumps is 250 gpm. Full load feedwater demand is 1226 gpm into 3810' TDH. Feedwater temp is 200 deg. F. Three pumps, two on-line, one 50% hot standby. This is a constant-head application, i.e. as steam generators are brought on and off-line, I still need 3810' TDH. There is not much friction loss at all across this system; the system curve is VERY flat. Control is by PLC. The only automatic controls present are the inlet feedwater rate control valves at the generators (Fisher E body) and the min bypass disch recycle valves on the pumps (CCI Drag valves). All other valves are 1500# manual gate valves (I know, overkill... don't get me started...).

These are oilfield steam generators. Feedwater source is an API 650 tank. There is no reheat cycle, no economizer, no deaerator, no steam drum, no condensate recovery, no blowdown, no nuthin'. Feedwater in, steam out, into the ground, and gone forever.

My question is thus. I am designing the pump switchover procedure. Suppose pumps A and B are online and I want to start C and shut down B, for whatever reason. The switchover procedurer is manual.

The Question: How do I start C without deadheading it and ramming it backwards on its curve?

As I see it, the procedure would be to:
(1) hit start button for C pump
(2) C pump completes its pre-start warm-up cycle
(3) block valve in C pump disch lateral is closed
(3) Motor starts and min bypass disch flow control valve opens to 100%
(4) pump is bypassing 250 gpm back to the feedwater tank

Now I'm stuck. If the operator starts to open the block valve on C pump, the system won't take the water so the pump backs up on its curve - yes? The min flow bypass is still open so 250 gpm is still going out through the bypass but no rate is going out the discharge becasue the system won't take it.

I might have just answered my own question. Am I missing something here? Will I back up C on its curve before B goes off-line? I am anticipating the switchover procedure to take about 10 minutes or so.

Thanks guys!
Pete

Thanks!
Pete

 

[KernOily]

Well this whole thing just became a moot point. Argh.

The owner has decided to go with an individual PD pump on VSD (Wheatley quintuplex plunger pump) on each generator, instead of one or two large pumps feeding the whole steam plant. To semi-hijack my own thread, here are the reasons why they decided this way. I thought you'd want to hear this.

• Maximum turndown capability to handle the steam demand swings that are seen in this area of the lease. One pump for one generator. Can turn down as low as one generator without wasting energy, whereas with the large centrifugals, can only turn the plant down to four generators (pumps will be at min flow at four generators) before opening the min flow bypass and running the pump back on its curve.
• Cost of repair after catastrophic failure is much lower, and shop turnaround time is much faster, for an individual PD pump vs. a large split-case centrifugals.
• They have successful prior installation of Wheatleys on variable-speed drive at other project areas.
• The frequent maintenance items required of a Wheatley (power end oil changes, packing adjustments and changes, belt changes, etc.) can be planned as part of the State-mandated 60-day shutdown/safety inspection for the generator, so the generator will not be required to shut down for Wheatley preventive maintenance, therefore there is no net negative impact on steam plant availability resulting from the higher maintenance frequency and tasks required of a PD pump vs. a centrifugal.
• Unplanned trip (ESD) of a large online centrifugal will knock down the whole steam plant (all generators) because the controls can’t be made to start the backup pump fast enough on a trip of the online pump, that is, the backup pump can't be brought online and up to speed before the generators go down on low flow. This is an unattended facility. (Not sure this one is completely true but they didn't give me any time to sort this one out...).
• Specialized maintenance skill is not required on the Wheatleys and it is locally available. Specialized skill is required for the split-case pumps and the nearest location for that skill is two hours plus away.

If the plant load was more constant and they didn't need all that turndown I would have pitched more of a fit to keep the split-cases.

This is a classic case of the requisite decision-makers not being present at the outset of a project, when this decision should have been made, rather than 2/3 of the way through detail engineering. So we have spent $50M-plus on engineering thus far on the large centrifugals, all for naught, it turns out. Well it was good experience...

Guys thanks for all your time and help with this. I am going to finish out the P&IDs for the centralized option (split-case pumps) but it will get shelved, at least at this point.

Thanks and have a great Thanksgiving.
Pete

 

[BigInch]

Actually it sounds like a very good solution, given the number of generators. As mentioned above a couple of times, the 2/3 pumps seemed short for 10 generators. They have a lot of valid points. And, it is a production field, so wells come on and go off quite a bit, so the variable load is most likely there and VFDs like that.

Be sure to say thanks for your 50 M blessings, put the smiles on and do it again Monday. Enjoy that fat little turkey while it lasts.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com