National Oilwell Plunger Pumps 2

[JRLAKE]

Hello everyone.

I have been stumped again, twice now in one month. We have Qty (9) National Oilwell 200 HP Plunger Pumps Model 200T-5M, installed on an extrusion application. Flow is 90 GPM @ 2900 PSI (80% Water/20% Oil mixture, clean and no abrasives). The accumulator pressure (suction side) is 25 PSI. The problem I am having is excessive wear within the stuffing box. It looks and feels as if there is relative motion between packing rings and stuffing box. The wear is radial, around the full ID of the bore, and is a bit wider than the width of the rings, as if the rings are shifting a centimeter or so back and forth with the plunger. The packing material is kevlar, notorious for wearing metal. The suffing box is 316SS, plungers are 416SS. There is no wear on the plunger. We are getting about 2 weeks worth of "seal" with the packing. After that, it leaks about 1 gallon / hour.

Here is the kicker. There are three extrusion lines with three pumps on each line, for the total of nine pumps. One extrusion line (three pumps) has a three year "seal" life. The other two extrusion lines have the problem I described above. Naturally I looked at the differences among the three lines. The major difference is the suction system. The good actor has a well designed suction. The bad actors have very poor suction designs, with several 90's and hard T's (4") within 6 feet of the pump suction. There are clear signs of cavitation with pitting on the valve seats.

My plan is to bore and chrome the stuffing box and switch to a different kind of packing, but I don't just want to bandaide a symptom of the real problem. That brings me to my long and winded point.

Question: Is it possible for cavitation effects to migrate back towards the stuffing box and cause the problem described above? I can't seem to justify this in my feeble mind.

 

[chief]

If this is a x-head type pump, check the alignment of the sucton /discharge module with the piston rod. This is done by removing the packing and using an inside caliper ,gauge in 4 places around the periphery of the stuffing box from the rod outwards. The symptoms suggest you may have a misalignment in this area.
The module is located by pinning to the block and is a major replacement by the OEM if there is misalignment.


Offshore Engineering&Design

 

[JRLAKE]

Chief,

Thanks for your suggestion. I had not thought of this and it is a very good idea. I will definitely check this out. If it was an alignment problem though, wouldn't the wear favor one side of the stuffing box?

 

[chief]

It should favor one side of the stuffing box in theory, but it does not always work out in practice. Another possibility is a bent piston rod or intermediate rod. I have had all of these symptoms with x-head pumps. Take your readings at zero, middle and full stroke positions. Also check the wear down clearance on your x-head slides, again at the 3 stroke positions. You may have several problems accumulating, to give your stuffing box troubles, It would be a good idea to check all your bearing clearances against the OEM recommended settings, main bearing,Big end, x-head pin etc.

Offshore Engineering&Design

 

[BillBirch]

Wouldn't cross-head misalignment manifest itself as excessive heat?

 

[chief]

yes , you will have increased heat generated if you have misalignment and of course increased wear in your stuffing box and piston /liners.
Cavitation is a sympton, usually related to centrifugal pumps. With reciprocating pumps , any problems on the suction delivery will be noticeable with the pump 'jacking', ie. sounding like a rock crusher.

Offshore Engineering&Design

 

[HEC]

JRLAKE,
I would tend to agree with your first assesment of suction problems, the rod packings would generally be restrained against a positive pressure in the cavity. If there is excessive negative pressure in the inlet stroke the outer portion of the gland is likely to move relative to the housing, consequence wear. Is it a sealed system and are you able to increase the suction pressure to the point where there is no cavitatoin of the two missbehaving units, this would be a simple test. If it don't look right it probably is not right!

Mark Hutton

 

[JRLAKE]

Thanks Mark. This is a great point. The design of this pump is a bit different than you picture however. The stuffing box is threaded into the head and the packing is spring loaded. This design also supports your theory, maybe even more so.

I checked alignment and they are all fine. I am installing an inlet stabilizer to see if this helps. If not, I am leaning towards a redo on the suction piping.

Thanks for all of your suggestions.