This technology is efficient and provides a steady, non-pulsating flow.
by Eric Gray (NOV Completions & Production Solutions)
October 6, 2015

Operators face many challenges when producing oil and gas. This includes the number of wells operating at any given time, the volume of sand, fluid viscosity, system pressure and more. These challenges result in process parameters that can vary substantially. One progressing cavity pump has a low shear pumping action with a steady non-pulsating flow, which accurately transfers hydrocarbons and other liquids under fluctuating operation conditions. The versatility of the pump has been highlighted at offloading terminals at produced water disposal sites.

Utah Case Study

A large oil producer in the Myton, Utah, area had process problems in their produced water treatment and their disposal facility with centrifugal pumps they were using and was seeking a solution. A pumping technology provider was invited to consider the problem and discovered two main issues. First, the pump was experiencing cavitation at the inlet. The low and varying net positive suction head (NPSH) available was causing the centrifugal pumps to cavitate, leading to monthly failures of the mechanical seals. These seal failures were costly and resulted in extensive system downtime to the Utah oil producer’s operations.

The second issue was the emulsification of the oily water being transferred. The transfer process was further delayed because of the long residence time required in the storage tanks to allow the larger residual oil droplets in the water to separate by gravity before the water could be transferred. To allow for emulsification, the fluid may have to sit in the storage tanks for up to two days just to separate. Eventually, the water was transferred into mechanical separators to remove additional oil droplets too small to separate by gravity before being injected into a disposal well.

The gentle pumping action of the progressing cavity pump does not further emulsify the oil droplets 
in the produced water, making it easier to separate downstreamImage 1. The gentle pumping action of the progressing cavity pump does not further emulsify the oil droplets 
in the produced water, making it easier to separate downstream. (Images courtesy of NOV Completions & Production Solutions)

Truck Offload Solution

Four progressing cavity pumps were installed to operate at two offload stations at the producer’s treatment and disposal facilities. The pumps can transfer up to 130 barrels of produced water from each truck to storage tanks. It takes approximately 14 minutes to offload each truck. On a typical day, the trucks are lined up to offload. Since the first two units started up two years ago, they have transferred approximately 12 million barrels of produced water without any downtime.

The gentle pumping action of the progressing cavity pump does not further emulsify the oil droplets in the produced water, making it easier to separate downstream. The ability to operate at varying inlet pressures is an important feature in this application. Centrifugal pumps perform at their highest efficiency at specific inlet and outlet head conditions. The offload pumps are typically installed in buildings at road level. So the inlet pressure head is constantly dropping as the trucks are unloaded. The head is constantly changing as the fluid is pumped off, with a range of several feet from start to finish. However, progressing cavity pumps are not affected by this change in the inlet head and gently transfer the fluid. The pump does not emulsify the fluid, so users do not experience extended wait time to allow for gravity separation in the storage tanks, which allows a more continuous fluid transfer into and out of the storage tanks.

It is a common practice for the truck drivers that operate the offload equipment to create a vacuum on the inlet of the pump during the last 35 to 40 barrels of water to offload. This is accomplished by closing the bleeder valve at the top of the truck while the pump is still running. When the truck is empty, the pump is turned off and the valve at the inlet to the pump is closed. Then, the hose near the valve is loosened, and the vacuum in the system pulls the residual produced water in the hose back into the truck. This prevents toxic produced water from spilling.

This practice of creating a vacuum causes even greater change in the inlet pressure. This change in inlet pressure further reduces the efficiency of centrifugal pumps and can cause cavitation and result in the failure of both mechanical seals. But one progressing cavity pump, with its one mechanical seal mounted on a hollow drive shaft, is not damaged by any of the vibration resulting from cavitation and operates smoothly under these parameters at a rotational speed typically around 200 rotations per minute (rpm).

Pump Design

The pump design consists of a single helical shaped metal rotor that rolls eccentrically in a double helical elastomer stator. When combined, sealed pockets are created, which move toward the discharge end of the pump as the rotor is turned. A progressing cavity pump is type of positive displacement pump, meaning that a fixed volume is displaced with each revolution.

These pumps provide operating efficiency, dependability and versatility in handling a wide range of applications. For handling clean hydrocarbon liquids and shear sensitive chemicals as well as viscous, corrosive, abrasive, solids-laden oily water slurries and sludges, progressing cavity pumps are an appropriate choice.

Flow through a progressing cavity pump is a function of rotational speed. If the fluid is shear sensitive, viscous or abrasive, running the pump at lower speeds improves efficiency and increases operating life. The elastomer stator allows frac sand and other abrasive particles to roll along the surface, minimizing scrapes and abrasions.

One progressing cavity pump uses a gear-type universal joint design, which effectively handles radial and thrust loads resulting from the differential pressure the pump operates against. The uniform design of the ring and ball gear, along with the thrust plates, allow the forces to be distributed over a much larger area than in pin or carden type joints. This generally results in less wear and better overall performance, but is especially beneficial in the unpredictable upset conditions.

Other features and benefits of this type of progressing cavity pump include a steady non-pulsating flow, which accurately transfers hydrocarbons and other liquids and is ideal for metering. The pump is self-priming, will not vapor lock and operates quietly without vibration that could disturb the calibration of other instrument nearby. It is available in various materials of construction, can handle flows up to 73,970 barrels per day (bpd) and can operate against differential pressures up to 522 psig.

Conclusion

By incorporating advanced technology, superior design and proprietary manufacturing processes, progressing cavity pumps offer an effective pumping solution for the difficult and fluctuating conditions often encountered in the oil and gas industry. The gear-type universal joint design effectively handles radial and thrust loads for maximum performance and long operation life. The pump’s rugged and reliable design is not susceptible to catastrophic failures and has proven to be the lowest total cost of owning/operating over the products life compared to other pump types.