Expendables such as valves, plungers, packing, pistons and liners are exactly that: expendable. Over time these items will wear out and will need to be replaced. This time frame can be very short or it can be relatively long. If the time is short, the owner is pouring money into the pump for parts and, even more importantly, costing him downtime. The ideal situation is to perform scheduled maintenance on the pump in your yard rather than let the pump dictate its repairs on the job. The contractor can have a major influence on the cost to operate a pump.
There are three primary areas to look at in determining expendable life: pump speed, suction conditions and suspended soils.
Speed is a direct indicator of expendable wear in reciprocating pumps much like that of a car. If you drive a small compact car at 100 mph (160 km/h) vs. a large sedan at 60 mph (96 k/h), it is obvious that it will require more maintenance to operate the small car.
Let’s compare two pumps. Pump A is a 1 fl-in. (44 mm) stroke triplex, and Pump B is a 2 fl-in. (56 mm) stroke triplex; both are sized to accomplish the same job. The units are set up for 15 gpm. The units are set up for 15 gpm (114 Lpm) on the backream. Each will deliver the same volume and pressure at different speeds. Pump A is running at its maximum capacity and Pump B at a lower speed. We will assume each pump will operate for a period of four hours on the pilot bore (15 gpm/57 Lpm) and four hours on the backream (30 gpm/114 Lpm), five days per week or 160 hours per month.
The following numbers reflect wear in miles on the plunger, piston, liner or packing the expendables encountered in a 20-day month–160 hours. The valve velocity expressed in feet per second and the repetitions (actual openings and closing) are up and down movements.
Wear on piston, liner, plunger or packing (back and forth movement)
Pump A — 226 miles (362 km)
Pump B — 166 miles (266 km)
Velocity through valves in feet per second
Pump A — 11.2 ft (3.4 m)
Pump B — 6.7 ft. (2.0 m)
Number of valve openings and closings
Pump A — 4,680,000
Pump B — 2,668,800
As can be determined by the above numbers, speed or pump stroke does have a lot to do with the wear expendables encounter.
Due to the necessity to keep a rig as small and compact as possible, the suction piping on a pump can suffer. The flow from the mud tank to the inlet of the pump needs to be big, short and straight. Bends and strainers only restrict the flow to the pump and can induce cavitation.
Because a single acting pump does not have the ability to lift liquid, the pump must rely on the available head (height of liquid) and atmospheric pressure to drive the liquid to the pump.
As the viscosity of the liquid increases it takes more time to move through the suction piping to the inlet of the pump. If the pump is running faster than the liquid can be delivered , the pump will cavitate. The velocity of the liquid in the suction line should never be less than 1 ft (.3 m) per second and no more than 3 ft (.9 m) per second.
From a pump manufacturer’s standpoint, any strainer in the suction is nothing more than a restriction. If one is used, the mesh should be large enough to stop a small pebble and nothing more. A function of bentonite is to find a hole and plug it. The bentonite will plug a fine screen or restrict flow enough to cause the pump to operate in a continuing mode of cavitation.
Now comes the easy part: solids. If one uses a a high quality bentonite and water, expendable life will certainly increase. Bentonite comes in different grades and qualities so it is important to use the lowest sand content available. Water is also an important part of th mix. To quote one of the mud representatives, “When you drive into a city and see a sign that states, ‘Public Water Supply Approved,’ that just means it won’t kill you–today.”
That is not necessarily true if you are a directional driller. If you hook to a fire hydrant and get a charge of sand in the mud tank, the plunger will probably die that day. The valve assemblies will also experience unnecessary wear.
There are a number of manufacturers that make a strainer to preclean the water to 50 micron before it ever enters the mud tank. This is one of the most inexpensive investments that a contractor can add to a rig. It will pay for itself again and again. The water should be filtered regardless of the source.
In the instances where mud recirculation, recovery and cleaning systems are employed, it benefits the contractor to ensure that the mud system can effectively remove solids as small as 15 to 25 micron. If not, premature pump expendable wear should be expected.
If one uses a properly sized pump, quality water and bentonite, and a good suction system, the overall cost of pump operation should be low.