Equipment Load Management in Plants & Refineries | Energy Conservation

Equipment Load Management – Managing the load on various items of energy-consuming equipment falls into the category of process operations optimization. The fundamental concept is to extract some operating cost savings in the form of reduced energy consumption from the capital that has already been invested in equipment assets, but is not being utilized due to the fluctuations in the production.

Equipment Load Management in Plants & Refineries | Energy Conservation

The objective is to operate the equipment at the lowest total cost while still meeting the process objective. Several general principles and strategies apply in most cases:

  • Minimize number of machines being operated in parallel. 
  • Reduce the rate at which individual machines are being run, through minimizing recycle flows.
  • Operate equipment at near its maximum efficiency point, to the extent possible
  • Assign maximum duty to the most efficient equipment (in a parallel set), and use the least efficient equipment as the “swing” unit
  • There is always a trade-off. The fewer the number of parallel machines that are running at any given time, the less redundancy you will be having with naturally possible loss of some operating flexibility. Next a one more note about drivers from load management point of view is introduced.
  • Pumps and compressors are usually driven by electric motors. Sometimes the motive power is provided by steam turbines (usually in the 500-10,000 HP range) or by gas turbines (usually >10,000 HP). Steam and gas turbines, on the other hand, are inherently variable speed devices.
  • The correct choice of driver – whether motor or turbine – depends on whether speed control would be beneficial in that particular application, and on the size (power consumption) of the pump or compressor. The overall site steam and power balance also has a considerable influence on the economics of driver selection (especially for the larger sizes), and should not be ignored.
  • It is well known that savings can be substantially higher for pumping systems with adjustable speed (variable frequency) drives on the motors.
  • Flow control can be achieved in many different ways – by throttling the main discharge line, by running the pump at full throttle and re-circulating the excess flow, or by using an adjustable speed drive. Flow recirculation is also employed for protecting the pump against mechanical damage that could occur at low-flow conditions.
  • Pump power consumption is a function of flow. Recirculation through the bypass line increases flow rate and wastes energy; therefore it should only be employed when the net process flow falls below the minimum flow requirement of the pump. The opportunity for energy savings arises when some flow is being re-circulated through the by-pass line even when it is not needed. This usually happens when the pumps are grossly oversized for the required service, a consequence of excessive conservatism during the project planning and design phase.
  • “PV” work is the useful energy absorbed into the process for increasing pressure or driving the fluid. However, a certain amount of input power is lost to heat due to friction. Observe that the pump efficiency (useful energy divided by input power) is not constant but in fact goes through a maximum over the pump’s operating range, falling off to near zero at extremely low flow rates.
  • To calculate the power consumption for each case (actual, required, minimum), use the average flow rate and head for that time interval. Pump efficiencies at the relevant flow rates should be obtained either from the pump manufacturer’s data sheet/curve or from the efficiency data generated during the most recent pump performance test.
  • The power savings for each time interval must be added up for all intervals during the year to get the total annual savings. The pump flow profile histogram is a very good indicator of whether there is significant cost saving potential from elimination or minimization of recycle.

The efficiency of pumps is a function of flow rate. Sometimes, the efficiency can be significantly lower at flow rates beyond the “best efficiency” point, and one has to check to make sure that minimizing the number of operating pumps will in fact minimize power consumption.

If this is not the case, then the operating policy developed for minimizing pump trains in operation must be revisited and revised as necessary. Real efficiency curves are seldom as extreme as the one shown in the graph below, but it makes the point.

In the case of recycle elimination/minimization, however, there is never a case to be made for operating at higher flow than the minimum, because the increased power consumption due to higher flow will always exceeds the savings from efficiency improvement.

So far we have assumed that all pumps in parallel are identical, and have identical efficiencies. In fact, this can never be strictly true, especially if one of them suffers mechanical deterioration at a faster rate than another. Having said that, the appropriate operating policy, when we have parallel machines of unequal efficiency, is both simple and obvious, use the most efficient machine for base load and the least efficient machine for swing loads.


 

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