Avoid these common problems for longer equipment life.
by Mark Nowell

The fluid end is a critical component of the pressure pumping industry. Fluid ends also can be the biggest heartburn for frac companies when they fail prematurely. Extreme environments such as 100 mesh frac sands, ever-increasing pressures, recycled water, advanced chemicals and slickwater all contribute to fluid end failure in as little as 100 to 500 pumping hours. In today’s market, that is an expensive problem. This article looks at five consistent failure points of the frac fluid end. Solving these problems can extend the life of the fluid end and reduce the cost per hour. Given the industry’s current climate, saving money and increasing profitability and productivity are a big win.

Failure 1: Fatigue Cracking

The most common failure of frac fluid ends is fatigue cracking. The simplest way to look at metal fatigue in high-stress applications is to focus on the endurance limit of metals, which is roughly half the material’s tensile strength. If stresses on the fluid end fall below that limit, it should have infinite fatigue life, meaning it should not crack.

The intersecting bore is the point of the fluid end that experiences the highest amount of stress. Most fluid ends that are machined and hand deburred have intersecting bores that begin to reach or exceed their endurance limits when stressed by a pump running at 10,000 pounds per square inch (psi) or greater.

Advanced geometry design of the intersecting bore can reduce stress below its endurance limit, but historically, fluid ends have been made of high-strength carbon steel that can corrode quickly. Corrosion on the surface of the material decreases the endurance limit by about 50 percent, which is why traditional steel fluid ends crack so easily. This phenomenon of stress corrosion cracking is clear—both in theory and in the field. Once the internal stresses of the fluid end exceed the endurance limits of the metal, it is a matter of time before the fluid end cracks.

optimized internal geometryFigure 1. Optimized internal geometry reduces stress experienced in critical intersecting bores. (Graphics courtesy of Kerr Pumps/FlowValve)

Fatigue cracking in fluid ends can be reduced significantly and often eliminated by constructing them from top-quality, high-strength stainless steel. Finding the right stainless steel that is both pure and tough is key to making better fluid ends. Superior stainless steel is usually free of hard spots and delta ferrites, or “junk,” in the material typically found in 17-4PH and even 15-5PH stainless. Fluid ends made with superior grade stainless steel are more cost-effective and reduce the cost per hour. Traditional fluid ends last only 100 to 500 pumping hours, but fluid ends made of superior stainless steel and optimized geometry can last nearly 10 times longer when properly maintained. That is 1,000 to 5,000 actual pumping hours, which is quite a return on investment.

Failure 2: Packing Bore Washouts

Properly maintaining fluid ends in high-cycle environments is not easy, since packing bores can wash out at any time. If a packing or grease system fails, the packing bore gets cut by the high-pressure water, causing it to lose its ability to seal. The use of stainless steel fluid ends achieves higher cycle hours, but with those longer hours of operation, the packing begins to wear into the packing bore, creating waves. This is called washboarding.

Eventually, the packing bore becomes so severely washboarded that the packing will not seal. Washouts and washboarding can be weld repaired, but that drastically reduces the strength of the fluid end because welded material cannot compare to the strength of forged stainless steel. Weld repairs lower the endurance limit of the fluid end at the surface.

Cracks in stainless fluid ends are found more frequently in welded areas than in non-welded areas.

box sleeveFigure 2. A patent-pending stuffing box sleeve no longer houses the seal on the outside diameter, transferring seal wear from the body to the replaceable sleeve.

The best solution for packing bore problems was invented many years ago when reciprocating pump companies began using removable stuffing boxes or stuffing box sleeves in their pumps. Stuffing boxes house the packing that goes into the fluid end, becoming a sacrificial piece.

Washouts still occur when using stuffing boxes, but instead of weakening the fluid end by weld repairs, transferring the wear to the replaceable stuffing box is a better solution.

Stuffing boxes and sleeves are a move in the right direction, but the issue of a solid seal remains. Because the outside diameter of the boxes and sleeves must seal on the inside diameter of the fluid end, the sealing area becomes washboarded and wears out the fluid end, just like the packing in the bore.

One company has developed a patent-pending technology to solve these sealing problems. Instead of using off-the-shelf seals that may not provide the best seal, they use engineered seals that fit perfectly into a groove machined into the fluid end. This new solution provides a more consistent and reliable seal while transferring the wear from the inside diameter of the fluid end to the outside diameter of the stuffing box sleeve. Now the stuffing box sleeve is a sacrificial piece in two places: on the inside diameter where the packing seals and the outside diameter against the fluid end.

sleeved stuffing boxFigure 3. The sleeved stuffing box fights washouts by becoming a sacrificial piece.

Failure 3: Suction Seal Bore

The sealing area around the suction seal bore is a common wear point in the fluid end. Companies that pump with 100 mesh sand see this often and early. The suction seal plug has a groove in it that houses a D-ring or other style seal, and the suction seal bore forms a seal against it on the front of the fluid end. As the pump vibrates and pulsates at high pressure, sand gets trapped in the sealing area, and the rubber moves against the fluid end.

Eventually, the seal wears so deeply around the seal bore of the fluid end that it cannot seal properly. This wear can also be a form of fretting fatigue that can initiate cracking. Typically, weld repairs are made to fix the fluid end sealing problems, but that also weakens the fluid end and decreases fatigue life because everything happens at the surface.

Companies have long tried to use locking mechanisms to keep the suction plug from moving back and forth, but that still does not solve the problem. A patent-pending technology exists that transfers the wear to the suction plug by adding a groove that houses the seal to the fluid end seal bore. This design’s suction plug is straight, without a groove. So, when pulsation and vibration rub the 100 mesh sand between the seal and the plug, the plug is the part that wears out. Because more than 90 percent of fluid end washouts occur in the packing and suction seal bores, sacrificing the replaceable parts is a much less expensive alternative to replacing the fluid end.

suction bore sealFigure 4. A patent-pending suction bore seal keeps with the theme of transferring the wear.

Failure 4: Valve Seat Washouts

Some companies change valve seats as often as every 40 hours, while some let them go up to 100 hours before replacing them.

Imagine changing valve seats on 16 pumps every 40 hours (a total of 160 valve seats). It takes hours and causes severe problems for the fluid end. Traditionally, valve seats are made of carburized metals. They are hydraulically pressed in with a metal-to-metal seal.

Because the hardness of the valve seat is almost twice as hard as the surface of the fluid end, wear problems often occur in the fluid end. Every time a valve seat is pulled out, a small amount of the material in the fluid end gets worn. Because the valve seats are changed in the field, dirt, grime and frac sand often get wedged between the seat and fluid end taper, leaving the taper scarred. This scarring prevents the valve seat from sealing properly, causing the fluid end to get washed out in the seat area.

A few companies have switched to valve seats made of tungsten carbide, one of the most wear-resistant materials known to man.

tungsten seatsFigure 5. Tungsten seats provide value by eliminating risky maintenance procedures.

Tungsten carbide valve seats can last five to 10 times longer than traditional seats, and that means a reduction in the risk of damaging the fluid end taper seat area. It also means that five to 10 times fewer hours are spent changing out seats. Moving to tungsten carbide seats is well worth the extra cost given the man hours saved and longer life of the fluid end.

Failure 5: Discharge Seal Bore

The last and least likely reason for fluid end failure is the discharge seal bore. It wears as it rubs against the seal on the discharge plug. This wear is not as bad as on the suction side because there is constant dynamic pressure and less pulsating and vibrating.

When valve seats get pulled to change out parts, the tools used can damage the seal bore areas.

There is a patent-pending technology in which the fluid end discharge bore houses the seal. This placement causes the seal to rub against the discharge plug instead of the fluid end and also protects the discharge bore from tools that could scar its sealing surface during maintenance.

universal sealsFigure 6. This patent-pending design uses universal seals for suction discharge and packing bores, simplifying maintenance and inventory.

Invest Wisely

Early fluid end failure is something every frac company experiences. The good news is that cracked fluid ends can easily be identified and eliminated or drastically reduced by focusing on these five solutions:

  1. Superior stainless steel. Fluid ends made of superior stainless steel extend fluid end life exponentially and drastically reduce cost per hour.
  2. Packing bores. New patent-pending technology provides engineered seals that are more consistent and reliable than off-the-shelf solutions.
  3. Suction seal bores. New patent-pending technology transfers wear to the suction plug so when washouts occur, the plug is the only thing to replace, keeping the fluid end surface intact and uncompromised. These new seals are a sacrificial piece to absorb the wear and extend the fluid end life.
  4. Tungsten valve seats. These can offer five to 10 times longer life, reducing downtime caused by frequent maintenance.
  5. Discharge seal bores. A patent-pending solution is available that makes the discharge bore the sacrificial part, keeping the bore areas from being damaged during maintenance.

Emerging technologies for maintaining and increasing the life of fluid ends are a game changer. Investing in these new materials and designs will extend the fluid end life from 100 to 500 hours to upwards of 1,000 to 5,000 hours.