Reliabilty of submersible pumps

[TNH]

For approx 500 kw sea water lift pump for an offshore application we are debating between submersible or vertical line shaft pumps. Will appreciate if end users who may have operated both types can throw some light on availability and reliability of these type of pumps. Also is 11 KV power supply a concern for submersible motor of this size.

 

[chief]

PLEUGER MAKE A RELIABLE SUBMERSIBLE PUMP. I HAVE EXPERIENCE WITH SHAFT DRIVEN PUMPS BY GOULD. THE SHAFT BEARINGS TEND TO WEAR AND CAUSE THE SHAFT TO WHIP. THE PUMP ITSELF IS OK BUT SHAFT DRIVES ARE A NIGHTMARE: THE BRONZE BEARING SPIDERS ALSO TEND TO CAUSE ELECTROLYSIS WEAR IN THE PUMP TUBING AT THE JOINTS ,FURTHER ADDING TO PROBLEMS WITH THE SHAFT ALIGNMENT:

 

[PUMPDESIGNER]

I have stayed off this topic because of a lack of direct experience. But ...

chief makes an excellent point about line shafts not being the end all. Many would prefer submersibles.

The only near experience I have is a quote I supplied to a company working near the Dead Sea. Stainless 316L submersible was their preference, and hey, they should know better than I.

PUMPDESIGNER

 

[d23]

TNH:

I have not looked at any (other) manufactures, but an 11kV sub motor I doubt exist. You will need a transformer. Do you have enough power to start across the line or do you need some type soft-start? Would harmonics be a problem for the rest of the platform if you use a VFD?

If you provide a little more info you would get better replies. Things like
1) H&Q
2) Operating environment (GOM, Dead Sea, Artic Ocean, etc.)
3) Reliability (what happens to the platform when it does fail)

Not trying to cause problems I just believe more information would give you better responses.

Good Luck.

 

[TNH]

Thanks to all for responding. I think I should give an update and clarify some of the questions raised. For the size of the submersible pump in question there are very few good vendors around. Both Pleuger (owned by Flowserve) and Weir have confmd. that 11 kV motor is not a possibility so if we go this way then we will have to step down. Soft starting is not preferable because on loss of one pump we would like to start the spare immediately - not doing so may risk tripping the compression platform on loss of cooling water. Direct online starting is prefered and is possible so we will go with DOL starting. Reliability of these pumps is important because they provide sea water cooling for the tempered cooling water system which is essential for running the platform. The site is in the middle east.

 

[drpump]

A few of things you should consider when comparing these two alternatives:
1) The use of several smaller submersible pumps, 4 or 5 vs 2, would give you a measure of flexability with regard to flow rate and multiple fractional backups in place of just one.
2) If you do choose the submersible option consider positioning sacrifical anodes (zinc bricks) around the pump to lessen corrosion problems.
3) Make sure you install a suction shroud around the motor to direct the flow past the motor to the pump to provide adequate cooling.
4) If you choose the line shaft option (or the submersible) make sure you specify the proper materials:
-316L flanged column & head
-Nitronics 50 shafting
-5' bearing spacing
-NickelAluminum Bronze bearing retainers w/ high durometer cutless rubber bearings
-Ductile NiResist bowls
-316L impellers
5) Using the lineshaft option affords you more latatude in driver horsepower selection and avoids the exposure of a submerged mechanical seal.

 

[d23]

TNH:

There are three or four petroleum based manufactures that provide the submergible type equipment you are looking for.

SLB.Com-Owns what use to be REDA Pump
Baker.com-Has Centrilift
Wood Group-Has ESP
ESP@Weatherford.com

Each of these companies would be willing to help with submergible designs. They have the equipment designed for this type application.

For what you are doing I would recommend you specify inconel shafts and a complete review of possible starting problems. When you start high HP units with a very short cable to the motor there is not much impedance to soften the blow. Some form of softstart may be required. This type of motor will typically be at FL RPM within 6 seconds. If you use something like a reactor start in may take 15 to 20 seconds to reach full load speed. That is still as fast as most convention surface motors.

Good Luck

 

[stanier]

Another manufacturer you might like to contact is Sulzer. trhey are providing pumps for the upgrade of the Goodwyn A platform in Western Australia. Ibelieve the materials are a super duplex stainless steel. From memory these are 1500kw units.