Gack Plunger Pump
This case study is about a three piston plunger pump used to pump “gack”. Gack is a term used to describe a substance that has a variable composition and is generally bad for the equipment it comes in contact with, particularly pumps. In this case the gack consisted of water and various unspecified and unknown petroleum products and byproducts with varying amounts of particulate.
The first time we saw the plungers was October 4, 2011. The set of three arrived with a NiCrBSi fused coating applied by others. We were told that the plungers had been in service for only a year. Here is what they looked like:
The scoring was from “mystery” particulate trapped between the plunger and the packing. This led to leakage and the need to repair them.
We ground off the old fused coating and found that one of the three had to be taken well below finished dimension in order to remove all of the original coating. In order the keep the price of the job within reason we first built up a layer of NiBSi 0.45 mm (0.018”) thick followed by a layer of tungsten carbide cobalt chrome (86-10Co-4Cr) 0.40 mm (0.016”) thick, which was finished ground back to 0.25 mm (0.010”) thick per side.
The other two were coated with only tungsten carbide and finished to a thickness of 0.25 mm (0.010”) per side.
Below are how they looked during and after processing, ready to go back into service:
That was the last time we saw them until March 24, 2016, when two of the original three were returned for re-servicing.
Here is what one of the two returned plungers looked like after 4.5 years of service.
The first plunger failed because the seal (packing) lost lubrication and the plunger overheated severely. This was the plunger that required a layer of Ni to reduce the thickness of carbide applied. The carbide failed at the Ni layer. The second plunger also lost lubrication but much later than the first one. The defect you see is from ‘gack’ under high pressure escaping from under the seal and eroding the coating much like high pressure water jetting.
The third unit did not have a lubrication failure and is still in service. Measurements of the failed plungers in areas where there is no erosion or coating failure due to overheating are IDENTICAL to the finished dimensions when they left our shop in 2011!
Zero wear suggests that if the lubrication system hadn’t failed these plungers would have been in service for many more years.
The sleeves that hold the packing that these plungers ride in have pretty much the same story. Two of the three failed and the one that didn’t have lubrication problems is still in service. We will be coating the ID of those two and they will become spares.
The two sleeves in for repair did have a little less than 0.001” of measurable wear in the area where the packing sits. The wear was caused by the packing getting loose and moving with the piston. There is also evidence of some ‘water jetting’ damage as well.
And the best part is that the coating of 86-10-4 that we are applying today is significantly better that the coating that was applied to these plungers. Back then the hardness was in the range of 1350-1375 Hv300, and today it is in the range of 1450-1500 Hv300.
