Use and abuse of pressure relief valves on positive displacement pumps
Over the past few months, I have visited several drilling operations where I have identified a very concerning lack of understanding of the function and operation of the pressure relief valve (PRV) that is fitted to circulation pumps on diamond core drill rigs.
On a deep-hole drill rig I noticed that the relief port of the pressure relief valve had been plugged with a pipe plug. I asked the supervisor why this had been done and he responded that he had put the plug in because the valve was leaking, Figure 1.
At a recent pre-deployment inspection of a drill rig, I saw that the pressure relief valve was installed with the discharge hose from the pump connected to the relief port of the pressure relief valve.
In a recent incident, the driller reported that the PQ drillstring became stuck in the borehole and in an effort to clear the borehole he increased the pump flow rate. He walked away from the drill but then noticed that the drill was slowly being lifted off it’s outriggers. The driller observed the circulating pressure gauge and noticed that it had gone past full deflection, (8 MPa). He then opened the by-pass hose to relieve the pressure and the rig dropped back to ground level. On inspection it was found that the adjusting screw on the pressure relief valve had been screwed fully in and so the relief valve could not operate.
Figure 1: Pressure relief valve with the relief port blocked off with a pipe plug.
In all these examples, the pressure relief valve was rendered inoperable and so a critical piece of safety equipment was rendered useless.
All the examples illustrate that the contractor, the supervisor and the safety staff had a very poor understanding of the purpose of the pressure relief valve. This note has been written to explain the these and other features of a pressure relief valve when installed on a positive displacement pump.
Why is a pressure relief valve essential on a drilling fluid circulation pump?
All circulation and foam injection pumps, on all drill rigs are positive displacement type pumps.
A positive displacement pump is designed to deliver a constant volume of fluid irrespective of the pressure against which the pump is pumping. This means that even if a blockage occurs in the circulating system and the circulating pressure increases, the volume of fluid flowing past the drill bit will be constant. This is essential in a diamond core drilling operation where rotational speeds are very high – if a blockage occurred and the volume of fluid flowing past the drill bit dropped, the bit may “burn in”.
While a constant flow of drilling fluid is essential, it also means that if the circulation system is completely blocked, the pressure in the system will increase until something happens to relieve the pressure; for example, a flexible hose may burst or one or more cylinder covers may be blown off the pump.
The function of the PRV is to relieve pressure in the event of a blockage in the circulating system and so prevent hoses from bursting or the pump being damaged. The PRV is therefore a critical piece of safety equipment and as such, it must be installed, inspected, tested, maintained, and repaired on a regular basis in accordance with a well-developed procedure.
Figure 2: A simple, spring type pressure relief valve
Although there are different types of PRV, all pressure relief valves used in drilling applications work on a similar principle. A simple pressure relief valve is shown in Figure 2.
This PRV operates with a ball and seat arrangement and a compression spring. Fluid pressure acts on one side of the ball while the compression spring acts on the other.
When the fluid pressure exceeds the pressure exerted by the spring the ball is pushed off its’ seat and fluid is allowed to flow out of the relief port thereby relieving the system pressure.
The tension on the spring can be altered by the adjusting screw and lock nut to allow the valve to relieve at a range of pressures.
Common problems with pressure relief valves
In addition to the examples of incorrect installation outlined above, there are other problems that can occur with a correctly installed PRV.
1. The relief pressure can be set too high, i.e. the adjusting screw can be turned in too far, or the relief pressure may be set too low.
2. If the drilling fluid system is high in solids content, i.e. drilled solids are being recirculated, solids can pack underneath the ball and valve seat and, over time, a thick layer of solids can be deposited. This can prevent system pressure being transferred to the ball and so render the valve inoperable.
It is important that the driller knows at what pressure the PRV is set to relieve, and it is essential that the valve is tested on a regular basis to ensure that it is in fact operable.
It is essential also that the driller knows when the pressure relief valve is relieving. When the valve relieves, drilling fluid is directed out of the relief port and so the volume of flow downhole will be reduced. If the driller continues coring with the valve relieving, there is a very good chance that the bit could be damaged or burn-in. It is always recommended therefore that a hose is run from the relief port to somewhere near the control panel where the driller can either see the fluid relieving or the driller can hear the fluid relieving.
Figure 3: Pressure relief valve installation with a relief port hose fitted.
These key features are shown in Figure 3.
What force is created when the circulation system is blocked
In the last example, above, the Driller reported that the drill rig was lifted off the ground when he increased the pump output – is this possible?
The answer this question, I would firstly like to set some context; the circulation system of a drill rig can be blocked in two places:
At the drill bit and,
In the annulus.
If the bit blocks, any increase in circulating pressure will be contained within the drillstring and so if the PRV is faulty, it is likely that pressure will be relieved by the waterswivel hose bursting, or the cylinder covers of the pump being displaced. A blockage at the bit cannot account for the drill rig being lifted off the ground.
If the blockage is in the annulus however, it is possible in certain cases that the drill rig could be lifted off the ground. The following explanation and calculations will illustrate this.
Almost exclusively, diamond core drill rigs are fitted with PD pumps manufactured by FMC, or copies of the FMC pump manufactured by pirate manufacturers.
Typically, these are W1122 series pumps that are rated to deliver fluid at a maximum pressure of 8 MPa (80 Bar or approximately 1180 psi). This means that if a blockage occurs in the circulating system and the pressure relief valve does not operate, the system will be exposed to drilling fluid at 8 MPa pressure.
Let’s consider the example that I mentioned earlier, the drill rig in question had a mass of about 8 MT:
A PQ borehole is 122 mm in diameter and so the cross-sectional area of a PQ drill bit is 11 700 mm2 or 0,0117 m2.
A pressure of 8 MPa acting on a PQ drill bit is therefore equivalent to a force of approximately 9,4 MT. It is easy to understand therefore how the drill rig could have been lifted off the ground.
If, however, the driller had been drilling BQ, the lifting force would have only been approximately 2,2 MT, and this is too low to lift the drill rig in question off the ground.
Summary
It is clear that the PRV is an essential piece of safety equipment and so the following recommendations are made:
1. At pre-deployment inspection stage:
a. The contractor must present a procedure to inspect, test and repair the PRV and the procedure must be interrogated and confirmed as practical and applicable.
b. The contractor must present a procedure to set the relief pressure on the PRV. The procedure must be interrogated and the ability of the driller and supervisor to conduct the procedure must be verified.
c. The correct installation of the PRV must be verified. Correct installation must include installation of a hose from the relief port to the driller control panel.
d. The PRV should be secured in such a way that it cannot be easily tampered with. Some mining companies insist on a locked cover over the PRV, and others require only a tag to indicate if the valve has been altered.
e. All cylinder cover studs and nuts on the circulation pump must be as per OEM specifications.
f. The installation of a by-pass system to relieve system pressure must be verified.
2. During pre-operation and periodic inspections:
a. The contractor must be able to state the pressure at which the valve is set to relieve, and the relief pressure must be checked at least once every 6 months.
b. The PRV must be inspected for leaks, damage, or tampering on a daily basis. If there is evidence that the PRV has been tampered with or the adjusting nut has been altered, the relief pressure must be tested and reset as required.