Upgrade Your Vertical Multistage LPG Pumps with DuPont™ Vespel® CR-6100
Vertical LPG pumps are a common source of headaches for hydrocarbon processing plants around the world. Shaft bushings wear out, seals fail, and the pump needs to be sent to the shop for overhaul. An upgrade of the wear components to DuPont™ Vespel® CR-6100 can make the pumps in this service much more reliable.
Why Vertical LPG Pumps Fail
Vertical multistage pumps are often selected because the service offers poor suction conditions. Installing a vertical pump lowers the elevation of the suction inlet, increasing the NPSHA (net positive suction head available), increasing the likelihood that the process fluid remains liquid when the pump operates. Even with a vertical orientation, suction conditions are often marginal. Light hydrocarbons easily vaporize, and it is quite easy for a vertical LPG pump to run dry at start up or require multiple attempts to establish suction. Vertical multistage LPG pumps also tend to be in transfer services which are subject to start/stop operation, which is tough on any machine.
Because of the risk of dry running, start/stop operation, and poor lubricity of the process fluid, the typical shaft bushing materials in vertical multistage pumps are bronze or carbon. Under dry running conditions, bronze can rapidly wear or potentially seize and create significant damage. Carbon generally will not seize, but it can wear, crack, and sometimes completely break apart, leaving the shaft without support. In this situation, the pump shaft can break, leading to expensive repairs and potentially hazardous failure modes. Even when these materials are not subject to dry running, the wear rate of bronze and carbon can be quite high, due to the low lubricity of the process fluid.
DuPont™ Vespel® CR-6100
Vespel® CR-6100 improves the reliability of vertical multistage LPG pumps by addressing the core challenges with the service. Vespel® CR-6100 can survive under dry-running conditions for significant periods of time. When operating in a low lubricity medium like LPG, Vespel® CR-6100 exhibits an extremely low wear rate.
In vertical multistage and vertical turbine pumps, Vespel® CR-6100 is used for the stationary wear components—line shaft bearings, bowl bearings, and stationary wear rings. The rotating parts remain metal. This upgrade eliminates the metal-to-metal contact interfaces within the pump, minimizing the risk of seizure. Because the risk of seizure is minimized, running clearances can be reduced. Depending on the configuration of the pump, wear ring clearances can often be reduced to 50% of the API minimum standard for metal wear rings.
This is a key upgrade. When wear ring clearances are reduced, the pump rotor becomes more stable, vibration levels are reduced, and the NPSHR (net positive suction head required) is reduced. What this means is that it will be easier for the pump to establish suction and less likely to run dry.
Vespel® CR-6100 has a unique set of material properties which make this possible. Vespel® CR-6100 is a composite material of Teflon® PFA resin and long carbon fibers. DuPont’s patented manufacturing process for the material produces a matrix of carbon fibers oriented in the x-y plane (the plane perpendicular to rotating in a pump). Due to the manufacturing process, Vespel® CR-6100 exhibits exceptional dimensional stability. Some of the key material properties are:
- Coefficient of thermal expansion lower than carbon steel
- Low wear rate
- High limiting PV
- Low coefficient of friction
Because the material ingredients are Teflon® PFA and carbon fiber, Vespel® CR-6100 is chemically compatible with nearly any fluid you might have in your process stream.
When assembling a vertical multistage pump, it is important to get the concentricity and alignment of the assembly correct. Because there are so many stages, slight errors at the top of the assembly can add up to large run-out at the bottom. Misalignment between the sections can induce shaft deflection and rotor instability. Improper alignment and field installation practices can create other significant problems. These issues will be addressed in part II later this year.