Understanding fluid end mechanics, a routine preventive maintenance program and high quality replacement parts are vital for ideal performance.
by Earl Foote, Southwest Oilfield Products, Inc.
March 27, 2013

Whether onshore or offshore, well drilling sites are highly complex work environments that rely on numerous systems to successfully perform the drilling operation. Among these systems, the rig’s mud pump is tasked with circulating drilling fluid (mud) under high pressure (up to 7,500 psi) downhole. The mud pump can be divided into two key sections: the power end and the fluid end.

Mud pump

The mud pump fluid end uses the reciprocating action of the power or drive end to create a suction and discharge flow that essentially pumps the mud down the drill string and back up the well’s annulus.

The key components of a mud pump fluid end are the piston and liner system, valve and seat system, and the fluid end module. As with most mechanical devices, any system malfunction can impact the performance of the entire unit or shutdown the pump completely.

Mud pump system

Having an in-depth understanding of fluid end mechanics, a routine preventive maintenance program and using high quality replacement parts are vital to keeping a mud pump fluid end performing properly. This article focuses on the importance of performing proper routine maintenance on mud pump fluid ends to ensure efficient operation.

Piston and Liner System

The piston and liner system of a mud pump fluid end consist of the piston, liner, liner seal, wear plate, wear plate seal, liner retainer, the piston rods and rod clamps. Of these items, the piston will experience the most wear and will typically be replaced the most. Routine piston maintenance presents an excellent opportunity to inspect other components in the system. For example, because of high levels of vibration during operation, rod clamps can loosen, which causes excess wear to the clamp and the rod. Additionally, a worn rod clamp can cause damage to the rod that would affect its alignment with respect to the centerline of the liner bore. Any misalignment of this type would cause accelerated wear of the piston and liner. In respect to addressing loose rod clamps, it is important to note that over torquing rod clamp bolts can have adverse effects as well. The recommended tightening torque for a rod clamp bolt is significantly less than the same size bolt used in a flanged connection application. A small gap should be between the two halves of a typical rod clamp when it is properly tightened.

Correct and incorrect tightening of the rod clamp

Over tightening the clamp causes the two halves to meet and runs the risk of overloading the clamp, which can lead to permanent damage of the rod and clamp.

Pistons generally provide longer service life when they are engineered for the environment in which they will operate. Therefore, operators must know the properties of the drilling mud that is being used as well as the drilling conditions. For example, if the mud temperature is more than 180 F, the operator needs to consider a piston that is designed for high-temperature service. The performance of all elastomers will degrade as temperature rises, but new elastomers are available on the market that will yield longer run times in high temperature environments. Generally, urethane pistons do not perform as well when running in water-based muds as nitrile or hydrogenated nitrile butadiene rubber pistons. Conversely, nitrile pistons do not perform as well in oil-based muds as urethane pistons.

Valve and Seat System

The valve and seat system consists of the valve, valve seat, valve spring, valve guide and bushing, valve cover, and valve cover seal. One of the most overlooked components of the fluid end is the valve spring. This inexpensive, small item can wreak havoc on the mud pump fluid end if it is not in proper running order. When the piston transitions from its suction stroke to its discharge stroke, the valve spring is supposed to push the suction valve down on the seat, closing the valve. A weak valve spring allows the valve to dwell in the open position longer than it should. This results in reduced efficiency because of mud flow reversal as well as the valve being slammed down on the valve seat by the returning piston stroke.

If a valve runs for 500 hours at 100 strokes per minute, the spring will experience 3,000,000 cycles. Considering its high cycling function, the valve spring should be replaced with every valve replacement.

As with all operations, safety is first and needs to be given a tremendous amount of consideration when performing valve and seat maintenance. The full open valve seat has become more popular in recent years due to improved flow and ease of maintenance. In valve over valve fluid ends, the full open valve seat allows easy access to the suction valve without having to remove the discharge valve seat. This was previously not an option with webbed seat designs. However, webbed seats are easier and safer to remove from the module because of their design and anchor points with the removal tools.

Split puller heads are used to pull full open seats. Split puller heads are designed to collapse to allow insertion into the full open seat. The puller screw is then threaded into the puller head to keep the puller head halves spread and allow the seat to be pulled.
A common problem arises when the puller screw is not completely threaded into the puller head. When attempting to pull the seat, the pulling action collapses the puller head halves and exerts a strong force on the puller head retaining ring resulting in a catastrophic puller head failure.

Seat puller head and screw, fully inserted  Seat puller head and screw, partially inserted

When this occurs, force is released, and the tool can severely injure anyone standing nearby. Standing clear when performing this operation is critical for personnel safety.

When replacing valve seats, care must be taken. The seat deck taper must be inspected for corrosive pitting. Areas of corrosive pitting can progress inward from the top and bottom of the seat. If they meet, they will create a fluid path for a wash to occur. When a valve seat becomes pitted, it must be lapped to remove the pitting. Some companies make tools for this. However, a simple way to do this is to use a three- or four-web seat as the lapping tool.

Another part that is often overlooked in the valve and seat system is the valve guide bushing. When performing valve or seat maintenance, the valve guide bushing should be inspected for excess wear or damage. The valve guide bushing should be replaced if necessary.

A worn or damaged valve guide bushing can cause the valve to stick or become misaligned and not engage the seat properly. This malfunction can lead to a module washout, a catastrophic failure of the module because of the high-pressure fluids.

Fluid End Modules

When a wash occurs in a fluid end module, little can be done in the field to repair it other than the lapping of the seat deck tapers that was previously mentioned. The module must be sent to a repair facility that is familiar with the proper weld procedure required to repair the washed out area. The repair shop must have a way to apply the proper preheating, post-weld stress relieving and the machining capability to perform suitable repairs to fluid end modules.

A plug seal washout

Keep an eye on valve cover seal areas. Normally, valve covers are not supposed to “breath” or move during operation. However, if a loose valve cover gland allows this to happen, wear in the seal area will occur. If left unchecked the valve cover will eventually leak.

Seals are an inexpensive item, and if an opportunity presents itself, a seal should be changed if an operator has the slightest doubt about its condition or age. All elastomeric parts have a shelf life, and heat and humidity will drastically reduce the part’s condition.
High Quality Replacement Parts

Along with performing routine maintenance, selecting high quality replacement parts is equally important because this has a direct impact on the overall performance of a mud system. As the old adage goes, “You get what you pay for.”

Depending on the drilling environment, some operators who have chosen to run lower quality replacement parts report a decreased life cycle of their consumable items and a reduction in the overall performance of their mud pump systems.

In conclusion, a disciplined preventive maintenance program in conjunction with using high quality replacement parts is crucial for proper mud pump fluid end performance.