Q & A > Question Details
I work in Hydrocracking plant, where we commonly use turbine pump when running in normal condition, and backed up by motor pump as a spare pump.
But, in some equipment, we use turbine pump as primary pump and backed up by turbine pump as a spare pump. This pump transfer the bottom of low pressure separator (liquid hydrocarbon) to debutanizer.
I also found a pump configuration where both the primary and spare pump are turbine pump. This pump is diesel pump around (hot wash).
Do you know what is the reason behind these configurations?
10/08/2010 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
There are multiple factors that come in to play in driver selection. Two major ones are relief scenarios and energy balances. Sometimes pump drivers will be selected so that in a given failure scenario, the loss of pumparound cooling is limited so that the total required relief load is limited to an acceptable level for your flare and flare header sizes. Sometimes high pressure to low pressure let down turbines are selected to help balance the steam system. If there is less low pressure steam produced than is needed to supply low pressure users like reboilers, then high pressure steam must be let down in pressure. This can be done across either a let down valve or a steam turbine. When you use a steam turbine the energy is almost "free", because all the energy not extracted by the turbine is used by the reboilers. The recovered energy in the turbine directly replaces electricity, where as steam let down across a valve wastes the potential for valuable power recovery.
As another reply mentioned, reliability of electricity supply vs steam supply can also be a factor. Sites that import power from an unreliable source will often maximize the number of steam turbines to minimize the effect of a power supply loss on plant operation. In this situation electric drivers may be selectively used minimize the impact of a steam supply failure on relief loads.
09/08/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Last thing first: Most diesel driven pumps use diesel engines, used in critical services where both electric and steam may be simultaneously lost, such as cooling water supply.
Within a hydrocracker, as in virtually any process unit, the use of motors and turbines for pump drivers varies from unit to unit. Whether there are steam turbines at all, as well as which is the primary and which is the spare. Each refinery location has its own situation in terms of the reliability of each power source, and judgments are made as to the best combination for both safety and service factor. Quite often, steam turbines on pumps are not installed simply because they are more expensive to purchase than electric motors. When this is the case, more frequent upsets are tolerated. If your turbines are the primary drivers, it was assumed, perhaps, that your steam system is more reliable than the electric power supply. Operating the motor driven pumps as primary usually results in a lower daily operating cost, but that would depend on the relative costs of any purchased gas (if the boilers are gas fired) and your cost of electricity.
Finally, within a unit, if turbines are the primary in some services, and motors are the primary in other services, it was probably based on an analysis by someone to minimize the extent of a particular type of upset. Opinions differ with such analyses; however, any measure to avoid reactor runaway is critical.