pump impeller material?

[upm]

Gents,
I need your advice for the recommended pump impeller material for duble suction between bearings tar pumps. the existing material is Ductil Iron and the inspection shows sever pin holes on the impeller blades and wearing ring areas. errosion marks on the blade tips (discharge) and sever errosion on the suction eye.
this is a very dirty service as the pump operates on cavitation, PH upset conditions
do you think Bronze will do the job , how it is compared with SS-410, do we need to upgrade the wearing rings as well?
thanks
upm

 

[cerz]

Generally, you need to use a harder material for both corrosive and erosive wear. In this case, you would do wel to use CD4MCu which is a duplex stainless. As foe the rings, they need to be of a harder material to avoid galling.
Your description sounds like you may also have some cavitation.

 

[upm]

thanks cerz
does duplex ss equevelant to ss-410 and 316?
we do have cavitation problems as well and abrasive wear is the result.
i was thinking to fit 316 impeller with ss-410 wearing ring. does it fix the problem?
thanks for the help

 

[kking]

If you use and inducer you can over come the cavitation problem also the introduction of air at the suction side will stop the pump cavitating.

 

[WGH]

Gents,

Although 316 is used pretty extensively in the pump industry it is still a relatively poor performer cf. to Duplex steel, particularly where corrosion / erosion / abrasion combined wear mechanisms exist. If you can afford the up front costs and downtime is an issue, go for Duplex.

 

[BobPE]

why is the pump cavitating upm? IT seems to me the easier fix, since the pump sounds destroyed, is to put in a better fit pump that doesnt cavitate. Provided that you have control of the tar viscosity, pumping tar is not a difficult thing.

I would think, describing the cavitation damage that you are, that even if you changed inpeller material, the pump casing and appurtenances would suffer the same fate. Changing both the pump casing and impeller material is a high dollar option...

BobPE

 

[upm]

Bobpe,
This is a very dirty service as the suction strainer keeps blogging frequently and as a results the pumps cavitate.furthermore, the suction piping lay out includes lots of bends (45deg) and the calculated friction loss is high. therfore the combination of friction loss and the process conditions have impeacted the pumps. upgrading the material will never solve the problem , however it will reduce the down time, this looks the cheapest option as purchasing new pumps or re-design the pipes cost is not compared with material upgrade due to plant shoutdown.

 

[BobPE]

upm:

thanks for the additional info it helps us a lot. why dont you try a relatively cheap investment in a suction pressure sensing system designed for tar. You can then alert maintenance people on low suction pressures and they can service the strainer. I dont feel comfortable specifying materials since the system will still fail after you repair it. It is a tough application, good luck...


BobPE

 

[scalleke]

What is the strainer mesh in front of the pump?

If the particles are small enough would it be possible to increase some clearances so the pump can handle bigger particles coming through?

This would help with the cavitation issue and at the same time a material change could help with the abrasion.

Maybe if the cavitation issue is gone the erosion like damage will reduce also.

I'm just thinking of some slurry type pump. Larger clearances less efficient but able to do the job for a longer time.

Best Regards.

Scalleke

 

[upm]

Thanx gents for the help,
the strainers size are 8 mesh or what we call it bolt catcher , it is big enough to allow smaller particles to pass in.however, overtime, it get accumulated and blocked. we do have dp. on the suction pipe as high dp across the strainer to alert maint.furthermore, we increased the wearing ring clearances to max. allowable and did improve alot. the main problem is the abrassive wear casued by the cock within the process?this is why i am thinking of upgrading the material
upm