High-pressure mud pumps are positive displacement pumps. Simply, they convert mechanical power into hydraulic power. Their mechanical components (power ends) are usually maintained by mechanics. The hydraulic part (fluid end) of the pump, however, is maintained by the drilling crew. As they are a critical part of the drilling rig equipment, it is mandatory that the drilling engineer should have a thorough knowledge of their mechanical and hydraulic components.
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The mud pump nomenclature, as given in Figure 1 showing a cross-section of a duplex pump, should be studied carefully to obtain knowledge of the basic terminology.
In this article, we will deal with:
- Working principles
- General construction
- Pulsation dampeners
- Relief valves
- Capacity, efficiency and required power
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Single Acting Pump
The power end of the pump serves to convert the rotary motion of the prime mover into a reciprocating motion. This reciprocating motion is, in turn, converted to fluid flow by a piston or plunger-type fluid end.
- Crosshead: The mud pump part where rotary motion changes into reciprocating motion
- The Connecting rod: It provides the linkage between the crank and crosshead.
- The piston or plunger: This is connected to the crosshead by the extension rod and the piston rod.
The main parts of the pump’s fluid end are: the housing itself, the liners with packing rings, covers plus packing, piston(s) with piston rods, and the suction and discharge valves with seats. (The discharge is normally supported with a pressure relief valve).
The plunger delivers during half the crank cycle only, for this reason, it is called a single-acting pump. We can calculate the delivery of a single-piston during one cycle of such a pump as following ( ignoring losses):
Double Acting Pump
The double-acting piston mud pump is shown in Figure 2. In this pump, there are only two pistons and the suction stroke on one side occurs at the same time as the discharge stroke on the other side. A double-acting pump delivers liquid both during the inward & outward piston strokes. So comparing it with a single-acting pump having the same piston diameter and stroke, it is clear that the double-acting pump will pump almost twice as much liquid. The output is obviously somewhat less due to the piston rod.
The main parts of the duplex mud pump fluid end are the same as those for the single-acting pump with the exception of the gland and stuffing box packing on the piston rod side, also the additional set of valves per piston.
Figure 4 is a view of the fluid end of a double acting duplex pump, showing major components.
Applications In Drilling
Both types of high-pressure mud pumps are being utilized on rigs for circulating the drilling fluid around. Each type, however, has its own operating limits, which are:
- Duplex pumps: the cranks are at 90° and the pump can be run at a maximum of 70 spm.
- Triplex pumps: the cranks are at 120° and can be run at a maximum of 150 – 170 spm.
In practice triplex pumps have the following advantages:
- The triplex pump provides a more even delivery so that discharge variations are about half those of a duplex pump. This provides a longer life of pump parts, hoses, etc.
- The total weight of a triplex pump is approximately 70 % of that of a duplex pump with the same capacity and it also requires less space.
- Triplex pumps are more accessible than duplex pumps and consequently, maintenance is faster and cheaper.
Downtime incurred due to pump repair is expensive and should be prevented. Frequent pump overhauls are demanding in terms of labour. Therefore the purchase of a generously sized pump is often preferred as this reduces the wear on parts, and the amount of maintenance required.
Common operational requirements may vary between 5.4 – 6.7 m3/min at 7,000 kPa (1,200 – 1,500 gpm at 1,000 psi) and 1.35 – 2.25 m3/min at 31,500 kPa (300 – 500 gpm at 4,500 psi). However, from experience it has been found that mud pump part wear sharply increases when operating the pump at pressures exceeding 21,000 kPa (3,000 psi).
Liner changes can be necessary when the flow rate ranges required during the course of the well cannot be covered by one and the same liner size, even when several pumps are run simultaneously.
Drilling Mud Pump Parts
The power and fluid ends are shown below in more detail.
Figure 05 gives a cut-away view of a complete power end. The rotating motion of a one-piece pinion and shaft is converted into a reciprocating motion by an eccentric shaft. All the shafts are supported by roller bearings.
The crosshead, which slides inside a crosshead guide to sustain the true linear movement of the extension rod is clearly visible as well as the stuffing box assembly which is also shown in detail in Figure 6.
The connection between the extension rod and piston rod is a fine tapered tread with jam nut (in a double-acting pump) or butted with a clamp (in a single acting pump) as shown in the figure below.
Figure 8 shows an example of the fluid end of a duplex mud pump.
- Item # 1 Fluid Module Only (Studded Included Items 1 – 6)
- Item # 2 Stud – Frame, Threaded Ring
- Item # 3 Stud – Discharge Manifold
- Item # 4 Stud – Threaded Ring
- Item # 5 Threaded Ring
- Item # 6 Discharge Manifold
- Item # 7 Threaded Plug Retainer
- Item # 8 Threaded Ring
- Item # 9 Cylinder Head Plug
- Item # Cylinder Head Plug – Top (Included Items 9 & 17)
- Item # 10 Threaded Liner Retainer
- Item # 11 Lower Valve Guide
- Item # 12 Retainer – Lower Valve Guide
- Item # 13 Seal – Cylinder Head Plug, Liner
- Item # 14 Discharge Manifold
- Item # 15 Seal – Suction Manifold
- Item # 16 Valve Guide Bushing
- Item # 17 Upper Valve Guide
Special attention should be paid to the oil flood stuffing box (Figure 9) which has to withstand the full pump pressure. The stuffing box is cooled and lubricated by an independent lubricating system.
A closed cooling system is used to protect the pistons of the single-acting pump from overheating (see Figure 10). The cooling liquid is usually a mixture of oil and potable water and is circulated by a small pump. If the pistons were not cooled the sleeves would become hot and overheat within a few minutes. The cylinder surface must also be lubricated.
The splash baffle on the extension rod prevents the cooling water being carried to the pump crank-case by the extension rod.
Piston and Liner
Liners of high-pressure mud pumps are always locked in place by a metal-to-metal contact.
A special liner lock nut is used in triplex pumps, duplex pumps are provided with a dual cage metal-to-metal liner retention system (Figure 11).
The latter arrangement holds the liner metal-to-metal against a shoulder in the housing by means of set screws), which are tightened against a liner cage. The liner packing is adjusted separately with set screws on a liner packing cage. Some types of pumps have special tell-tale holes (a small hole connecting the liner packing spacer ring area to the atmosphere) to check the proper functioning of the packing. If fluid drips out of these holes the liner packing have to be tightened. These holes should never be plugged off to stop the “leaking”!
Liner wear is worst in the middle of the stroke as a result of the piston velocity being highest at that point.
The maximum allowable liner wear depends on the pressure the pump has to overcome (see Table .1).
Figure 13 shows the pistons of a double-acting and a single-acting pump. Note that the piston bodies have been provided with a special tell-tale wear groove to provide a means for judging the piston wear.
The piston rubbers are made of polyurethane. The clearance between piston and liner determines the life of the rubbers (see Figure 14). If the play is too great it is possible that the rubber will be extruded into the gap and become torn.
Valves and Seats
The volumetric efficiency is strongly influenced by the condition of the valves. This means that regular inspection is a necessity if the efficiency is to be kept optimal.
After inspection the valves should always be returned to the seat from which they came. Valves and seats tend to wear together with matching wear patterns. They will give longer service if they are kept together.
After renewing a seat the valve should always be replaced. The valve is equipped with fins or a guide pin to ensure good alignment in the seat. A valve spring helps to close the valve.
Most of the valve seats are tapered on the outside to fit tightly into the pump body. It is essential that the mating faces are thoroughly cleaned, as improper seating could lead to a washout in the body which could spoil the entire fluid end.
As the fit of the valve seats must be very tight this means that the valves have to be driven into position with a copper bar and removed with a special (hydraulic) pulling tool.
Mud Pump Capacity, Efficiency, And Required Power Calculations
Mud Pump Capacity
As shown under (Working principles) the theoretical output per stroke is given by:
Theoretical output per stroke for a single acting triplex pump :
Theoretical output per stroke for a double acting duplex pump: All in consistent units
Multiplying the theoretical output per stroke by the recorded strokes per minute and adjusting for the volumetric efficiency (ηv) will give the effective output, Qe.
Thus, Theoretical output for a single acting triplex pump Qe =
where, in the two equations where units are given, L, D and d are expressed in inches, which is the unit normally used for this purpose.
Similarly, Theoretical output for a double acting duplex pump Qe:
where in the above equations
Pump Stroke Counters
Mud pumps are normally equipped with pump stroke counters. There are two types of stroke counters, one to indicate the pump rate (spm) and one to record the cumulative number of strokes. The latter type of counter is used to monitor jobs such as chasing Casing Cementing Job and spotting slugs and during well control.
In general, such counters are needed to follow the volumes pumped when volumetric control is essential. However, it is also necessary to know the pump efficiency to be able to determine the actual volume delivered. Given that this efficiency is pressure-related it should always be checked when pumping with reasonable pressure.
Volumetric Efficiency (ηv)
As stated in the previous paragraphs, the volumetric efficiency is the relationship between the theoretical and effective output of a pump.
Loss of volumetric efficiency is mainly caused by the delay in valve shutdown. When the plunger motion reverses the valves are not yet completely closed, due to the mass-inertia of the valves, and some of the liquid has the opportunity to flow back.
Major Causes Of Mud Pump Efficiency Drop
The following losses are recognized:
- Leakage losses of the discharge valve: As long as the discharge valve does not close completely during the suction stroke a small amount of liquid will flow back from the discharge line into the cylinder.
- Leakage losses of the suction valve: As long as the suction valve does not close completely during the discharge stroke a small amount of liquid will flow back from the cylinder into the suction line.
Other causes of loss in efficiency are:
- Losses due to a leaking stuffing box: During the suction stroke air is sucked in through the stuffing box. This air obviously reduces the overall suction volume of the pump. During the discharge stroke liquid will leak through the stuffing box so that the quantity discharged is also reduced.
- Leakage losses between piston and liner: The seal between the piston and liner may not be perfect, consequently during the discharge stroke some liquid may leak past the piston. It is also possible that during the suction stroke of a single-acting pump some air is drawn in past the piston.
- Leakage losses in suction lines: Leaks in the suction line may result in air being pulled into the drilling fluid flow during the suction stroke.
- Air or gas absorbed in liquids: The liquid itself may contain gas or air either dissolved or transported as small bubbles. One of the most common causes of suction aeration is the mixing of drilling f luid or the adding of chemicals through the hopper.
The highest practical efficiency should be maintained by regularly checking and servicing the pump.
Determination of The Pump Efficiency
The volumetric efficiency of a pump can be determined by pumping a known volume of þuid from one tank to another and comparing it with the theoretical volume calculated from the number of strokes made. This should be done whilst pumping over the well at a reasonable rate to ensure the pump is delivering against pressure (a good time to do this is when circulating prior to running casing and cementing).
The volumetric efficiency of a duplex pump will usually be 90 % or more. A triplex pump will usually have a volumetric efficiency greater than 95%.
Hydraulic Power Requirements
The hydraulic power (Ph) can be calculated using the equation below
Ph = ΔP x Qe
In Field Units:
Ph (HHP) = ΔP x Qe / 40
Ph is in psi and Qe is in bbls/min
In practice, the input power supplied must be greater than the hydraulic horsepower because of
- the work required by the mechanism of the pump itself.
- the work absorbed by the hydraulic inefficiencies
The ratio of the hydraulic power Ph to the input power Pi is the mechanical efficiency ηm, Thus
Ph = Pi x ηm
ηm is usually of the order of 0.85
Operating limits are set in the first place by the fluid end dimensions but ultimately by the power end. The maximum discharge pressure is determined by the size of liner installed, and the available torque from the power end. The output volume for a given liner depends on the attainable pump speed, i.e. the available power from the power end.
Mud Pump Relief Valves
Mud pumps must be equipped with pressure relief valves. These valves prevent too high a pressure being built up in the circulating system. The relief valves most commonly used in Shell operations are the Cameron type “B” reset relief valve and the Cameron shear relief valve. A discharge line should be connected between this valve and the drilling fluid tank. For safety reasons this relief valve discharge line must be tied down securely with its end facing down into the tank.
Cameron Shear Relief Valve
The Cameron shear relief valve pops open when pressure setting is exceeded. The tripping pressure of the valve is determined by the strength of the shear pin. In this design the valve snaps fully open and there is no erosion of the piston or bore. A chart is printed on the manufacturer’s name plate from which it is possible to see which size of shear pin is needed for the required pressure limitation.
Cameron Type “B” Reset Relief Valve
The Cameron type “B” reset relief valve provides the following features:
- It opens fully when pressure is exceeded.
- The pressure setting is indicated by a pointer.
- The pressure setting can be changed with pressure on the valve, by turning an adjusting nut.
- The valve design prevents leakage or erosion.
- All parts are enclosed.
- The valve is set by a reset lever.
- The valve can be opened at any time by pressing a release button.
The diagram below shows the valve’s operation.
Mud Pump Pulsation Dampeners
The speed of the piston is not constant during the suction and discharge strokes. During each stroke the speed increases from zero to maximum at approximately the halfway position and then decreases to zero during the rest of the stroke (see Figure 20).
The change in piston speed, and therefore also the fluid velocity, causes the oscillating action shown in Figure 21. The effect is less severe for a single-acting pump than for a double-acting pump.
The Suction Dampener
During the suction stroke when the pump requires more liquid it draws this from the dampener. Once the suction stroke has been completed the air chamber absorbs the flow from the tank and in this way dampens the shock in the suction line.
The Discharge Dampener
The discharge chamber or pulsation dampener, contrary to the suction dampener, is partly pressurised with nitrogen gas. During the discharge stroke the gas in the pulsation dampener is compressed. At the end of the discharge stroke the compressed gas expands sustaining a reasonable steady flow in the discharge line and dampening the peaks in discharge pressure.
Figure 22 shows a commonly used pulsation dampener. It consists of a steel spherical body in which a diaphragm is fitted. The diaphragm separates the gas (nitrogen) from the drilling fluid.
A charging valve and a pressure gauge are installed on top of the pulsation dampener cover to allow regular inspection and recharging. To achieve a satisfactory dampening effect the pre-charge pressure should be 75 % of the minimum anticipated pump operating pressure. The maximum pressure should not exceed 5250 kPa (750 psi).
WARNING: It is of the utmost importance that nitrogen only is used to charge the pulsation dampener. Serious accidents have been the result of using oxygen instead of nitrogen.
The best dampening effect is achieved when the dampeners are installed close to the pump’s suction and discharge, as shown in Figure 23. A hose to absorb vibration should be included in the connection between the pump and the delivery line.
Booster / Charge Pumps
When the pumps are running at the high end of their speed range even the suction dampeners may not be able to cope with the peak intake rates. This results in cavitation with the cylinders not being completely filled and shock loads in the pumps. To eliminate such problems booster or charge pumps are hooked up to the mud pump suction lines in order to maintain positive pressure at all times.
Attention must be paid to the following points to ensure that the pump cylinders are filled correctly to prevent piston hammering or pressure surges:
- The pump suction has to be as low as possible in relation to the suction tank so that a positive fluid head can be maintained.
- The pump has to be as near to the tank as possible so that the suction resistance is minimum. A booster pump in the suction line may be required (see Figure 3.3.24) if friction losses are excessive.
- The suction line must have an internal diameter as large as possible, and the line must be well sealed and secured to prevent air being sucked into it.
- The tank has to be kept full to the normal operating level so that the maximum head is maintained on the suction.