Challenges for pump design and specification in low temperature environments

Challenges for pump design and specification in low temperature environments

With the oil & gas projects moving more and more to severe environmental site conditions, the rotating equipment specialist is facing multiple and complex challenges to ensure safe and reliable operation using centrifugal pumps.
 
By Elise Boita, Lead Rotating Equipment Engineer & Alfredo Mastropasqua, Lead Rotating Equipment Engineer, Technip France

For the specific case of low ambient temperature, one might ask if the available standards applicable to pumps in the oil & gas industry, such as API 610, provide sufficient guidance to mitigate the adverse cold climate effects. Unfortunately, none of those standards is conceived specifically for such severe conditions, and new concerns introduced by low ambient temperatures need to be analyzed and faced for all applications. The most adverse effects of cold climate are icing and freezing, which may result in various detrimental phenomena such as additional weight loads, physical obstruction, change of material properties, freezing expansion damage, and electrical components failures. Cold climate effects on centrifugal pump design and operation must be carefully assessed at first place since any carry over work may drastically impair project execution. This paper presents few critical design considerations that should be investigated in equipment specification.

Material selection

API 610 provides a practical guideline for:

•    Selecting pump material according to operating conditions and pumped fluid
•    How to proceed and when to specify an impact test on pump casing when temperature is lower than 40°C
•    For carbon steel and low alloy steel, pressure containing parts with temperature down to -30°C, thickness governs the choice for impact test

When temperature is lower than -30°C, an extent of impact test needs to be carefully reviewed and identification of all components that were possibly exposed to low temperatures and brittle failure shall be performed. This review should cover all parts exposed to ambient temperature, hence structural components such as baseplate and bolts could require impact tests.

Start-up and driver sizing

Warm-up lines are generally used for specific service where automatic start-up on stand-by equipment is required. The addition of warm up lines cannot be generalized on all pumps in the plant, so the impact of minimum ambient temperature shall be taken into account. Even with heat conservation means in place on process lines, fluid viscosity increases when temperature drops from normal operating temperature to minimum ambient temperature. Viscosity effect could lead to considerably increased pump absorbed power. According to start-up conditions (open or closed discharge valve), start-up case shall be clearly identified and analyzed at early stages for correct driver sizing. Typically, electric motor sizing is done based on table 12 of API 610, and when open discharge valve start up is specified, the motor-rated power shall be sufficient to ensure pump operation at the end of the curve. In cases of high viscosity, driver oversize may be required to cover the start-up case.

Lubrication

Suitable lubrication of pump and driver bearings is a critical consideration to ensure usual maintenance intervals and package design life. Loss of lubricating properties at low or high temperatures can lead to irremediable bearing damages affecting the availability of equipment. The challenge is how to maintain oil and grease lubricating characteristics on a wide temperature range. Some solutions like the use of space heaters in bearing housing are already implemented for standard ambient conditions. Space heaters in bearing housing is already required when minimum ambient temperatures are below 10 °C, so they are typically put in place any time minimum ambient temperature is lower. The lubricating oil shall be selected among lube oils which allow keeping oil stable lubricating characteristics through the whole temperature range. When pressurized lube oil system are used, then all lines are heat traced and the size of the lube oil tank heater shall be done according to the most severe ambient temperature. The same approach shall be used for grease selection when grease lubricated bearings are used. This approach shall be followed of course for pumps, but also for motor drivers as well as for any other rotating auxiliary equipment.

To read the full article please contact the editor, Deirdre Morgan.

 

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