In a couple of recent posts, “Troubleshooting tales from the trenches” and “The baffling, ballooning cylinder,” Fluid Power World contributing editor Carl Dyke enlightened us about some problems that cropped up in the field. Here’s another interesting narrative of things that can go wrong, from Carl’s presentation at the 2017 Fluid Power Technical Conference in Milwaukee.
This one involved a high-pressure hydraulics application, a system that is rated to go up to 6,000 psi. Those of you in the audience probably work on a lot of systems that top out at 3,000 or 3,500 psi, so 6,000 psi stuff gets a little scarier. And this piece of oil-field equipment involved a 1.5-inch line that blew off twice within three or four weeks of each other.
Operations in the oil field can be a bit different, compared to other industries. If you’re coming from the construction machinery or agricultural machinery mindset, you might be producing 70 or 100 machines at a time. And you take a lot of time to do a lot of careful product testing.
Well, in the oil field there are a lot of one offs and two offs and sometimes they’re not happy with its performance. So they make engineering changes right on the machine. In this particular case, they went from a closed-loop hydrostatic system to an open-loop hydraulic system. And they put in a proportional directional valve, instead of stroking the swash plate forward and backward on a hydrostatic pump.
They had been working with a pump that had an enormous number of pressure protections built into the pump. These advanced pumps are, fluid-pressure wise, themselves almost a complete hydraulic system.
But in this case, the internal engineering department at this oil field company just sent engineering notes to the field on how to do the retrofit. And they specified no pressure relief valves at all between the hydraulic motor, which is about 150 feet away from the pump. The proportional directional valve is mounted close to the pump with the 150-ft hoses running out to the hydraulic motors—two lines in parallel for supply and return.
Right away when I arrived at the site they wanted to blame the hose manufacturer. They said, “It’s this new hose we switched over to.” Well, we cautioned, don’t jump to conclusions just yet. So we did crimp tests and measurement tests and checked for skiving. The other end of the hose was still intact so we cut that off and studied it. There was no reason to believe that the hose-fabrication shop had done anything wrong. In fact, the crimp diameter of the fitting connected to the valve was still exactly on spec. The other end of this hose that didn’t blow off was slightly overcrimped by about four of five thousandths. We thought, “Okay, that’s not too bad.” It was assembled as intended, so there was no reason for us to suspect the hose.
We did a fair amount of investigation on this one. In particular we talked to the operator about how the machine is used, how it runs, and how well it performs. And in passing he mentioned they had made some recent modifications. So we dove into the engineering details of the circuit retrofit and thought, “Where are the work port reliefs for the hydraulic motor, and where is the system relief?”
They were simply relying on the swash-plate destroke time on a very, very large open-loop pump—which, as you know, can take up to 40 to 50 milliseconds to move a swash plate all the way to zero in some of the larger pumps. And during that time, if you have a pressure spike coming back from the actuator—well, 40 or 50 milliseconds is a long time in pressure-spike land. And plenty enough time to blow that hose right off. So in their case, the simple requirement was to put in at least work-port reliefs for the ports between the directional valve and the hydraulic motor. And to have a system relief as well, that could catch the momentary spikes while the pump is in the process of destroking.
It was kind of funny, but also scary. Funny when the manager said, “I’m glad everybody’s here to help us investigate this. I’m tired of paying the environmental cleanup bills out at the lease for the well site.” And I thought, “The environmental bills? What would have happened if someone was standing anywhere near the machine when this happens—as it did twice in the period of about a month!”