How to seal rotating shafts for low temperature to cryogenic fluids.
by Mark Savage
June 28, 2018

Pumping fluids at extreme temperatures presents many challenges to the shaft sealing system. It must ensure the pumped fluid is safely contained while providing long-term reliability. Many pump operators are familiar with hot pumping services, but cold services offer some unique challenges to achieving reliable shaft sealing solutions.

Pumping Equipment for Low Temperature Hydrocarbons

The pumps used to pump these cold fluids are often specialized designs. Low temperature hydrocarbons are commonly pumped with American Petroleum Institute (API) 610 (VS6) vertical multi-stage double casing pumps that feature a warming chamber, known as a cofferdam, that thermally isolates the shaft seal from the cold pumped fluid. This enables a greater range of shaft sealing solutions that can be used on these pumps using traditional sealing technology.

A cofferdam is a chamber between the pump discharge and the mechanical seal that is connected to the pump suction or the vessel the pump is drawing suction from. Ambient heat surrounding the pump together with energy from the shaft and bearings causes the liquid in this chamber to vaporize into a gas, which forms an insulating barrier between the seal and process fluid. Cofferdams can only be incorporated
into vertical pump designs.

However, although vertical arrangements are common, various horizontal pumps can also be employed. In these types of pumps, the shaft seal is in direct contact with the cold pumped fluid, thus selection of the seal materials for low temperature operation becomes more critical.

Seal Selection for Low Temperature Hydrocarbons

Due to the volatility and flammability of low temperature hydrocarbons, dual mechanical seals are used almost exclusively.

image 1 vertical multistageImage 1. Typical vertical multistage (VS6) pump with a cofferdam (Images courtesy of the author)

For pump designs where the mechanical seal is immersed in the pumped fluid, vapor pressure margin in the seal chamber becomes critical. Where the vapor pressure margin is low, the heat energy from the mechanical seal faces can vaporize the fluid around the seal and in the seal interface, resulting in dry running of the seal. Seal chamber pressure can be increased through various flush piping plans to overcome this issue. However, when insufficient vapor pressure margin remains for the selected mechanical seal technology, a dual pressurized seal is recommended. A dual pressurized seal provides a stable barrier fluid to lubricate the seal faces, thereby negating the effect of vaporization of the pumped liquid at the seal faces.

API Plan 53B and 53C barrier systems are commonly selected for dual pressurized seals to provide a source of warm, clean and stable barrier fluid to the mechanical seal. When an API Plan 53C is selected, extra care should be taken to ensure the pressure amplifying piston seals are insulated from exposure to cold temperatures.

The availability of suitable barrier fluids becomes limited at low temperatures as the viscosity of many fluids becomes too high at the seal chamber operating temperatures.

image 2 crygenic sealImage 2. Typical vaporizing liquid single noncartridge cryogenic seal

Monoethylene and diethylene glycols mixtures with water can be used down to temperatures of -29 C (-20 F). Alcohols, such as propanol (propyl alcohol) are suitable for even colder temperatures reaching -70 C (-95 F). Synthetic oils can also be used. However, careful consideration to their pour point is required, and a heating system may be needed to warm the barrier fluid to maintain a suitable viscosity.

When sufficient vapor pressure margin exists within the seal chamber, a dual unpressurized seal can be selected. These designs feature a dry sliding containment seal fitted with API Plan 76 or a combination of Plan 72 and 76.

These seal arrangements have the advantage of removing the low temperature limitation of barrier fluid selection.

Pump designs using cofferdams require a dual pressurized mechanical seal.

Icing caused by condensation of atmospheric humidity can create a problem for cold hydrocarbon services. Because condensing water expands as it freezes, it can interfere with the operation of the mechanical seal if it gets to the seal’s operating mechanism. Extra protection considerations should be applied to equipment exposed to atmospheric elements. An API Plan 62 using a dry nitrogen quench can protect the mechanical seal from these effects.

Material Considerations for Low Temperature Hydrocarbons

Low temperatures have significant implications to the choice of materials used in the seal construction. This is especially true for elastomers. Elastomers have a variety of minimum temperature limits, but none can survive dynamic operation at true cryogenic temperatures.

Engineered polymer seals are an option at temperatures below the limits of elastomers. Many of these designs will not function with pressure reversals applied to the sealing ring that may be required in the mechanical seal design when support system failures occur.

Elastomers can survive at significantly lower temperatures below their operational limits when the seals are not in operation (i.e. static), but they must be warmed up prior to operation. Commissioning of shaft seals containing elastomers must be completed carefully to ensure that the equipment is at the correct temperatures before startup. Blowdown, the rapid depressurization of a vessel/pipeline, is one situation that can create excessively low temperatures for the mechanical seal’s elastomers.