With rising energy costs, process plants are increasing their focus on the amount of energy expended by rotating equipment. Improperly sized or poorly performing pumps are costing companies millions of unnecessary dollars. Unscheduled repair and poor reliability are causing companies to lose production and spend money on maintenance. Life cycle cost (LCC) analysis is a management tool that takes into consideration the costs to purchase, install, operate (including energy costs), maintain and dispose of all components of the system. Simply put, LCC analysis is the process of evaluating the overall cost of equipment ownership, maintenance and operation over its full life, rather than simply based on purchase cost alone. In many typical pumping systems approximately 50% of the total LCC stems from the energy costs. As the pump operation moves away from the best efficiency point (BEP), the energy costs will increase and the expected life of the pump will be reduced.
Improving energy efficiency
The energy cost is the largest element in the total cost of owning a pump. Depending on the industry, centrifugal pumps consume between 25% and 60% of a plant's electrical motor energy.
Proper matching of pump performance and system requirements, however, can reduce pump energy costs by an average of 20% in many cases. The process of specifying the right pump technology for an application in a given facility should go well beyond the initial cost but, in too many cases, it does not. Such a short-sighted approach can create major, long-term problems for organizations.
In the process industries, the purchase price of a centrifugal pump is often 5–10% of the total cost of ownership. Typically, considering current design practices, the LCC of a 100 horsepower (HP) pump system, including the costs to install, operate, maintain and decommission, will be more than 20 times the initial purchase price. In a marketplace that is relentless on cost, optimizing pump efficiency is an increasingly important consideration.
Once the pump is installed, its efficiency is determined predominantly by the process conditions. The major factors affecting performance include the efficiency of the pump and system components, overall system design, efficient pump control and appropriate maintenance cycles. To achieve the efficiencies available from the mechanical design, pump manufacturers must work closely with end users and design engineers to consider all of these factors when specifying pumps.
The vast majority of pumping systems run far from their BEP. For reasons ranging from short-sighted or overly conservative design, specification and procurement to decades of incremental changes in operating conditions, most pumps, pipes and control valves are too large or too small. In anticipation of future load growth, the end user, supplier and design engineers routinely add 10–50% ‘safety margins' to ensure the pump and motor can accommodate anticipated capacity increases.
Operation at high flows
Under these circumstances the head capacity curve intersects the system head curve at a capacity much in excess of the required flow, using excess power. Of course, the pump can be throttled back to the required capacity and the power reduced somewhat, but if the pump is uncontrolled it will always run at excess flow. Unless sufficient NPSH has been made available, the pump may suffer cavitation damage and power consumption will be excessive. Important energy savings can be made if, at the time of selecting the service conditions, reasonable restraints are exercised to avoid using excessive safety margins to obtain the rated service condition. If the pumps in an existing installation have excessive margins the following options are available:
1) The existing impeller can be reduced in size to meet the service condition required for the installation.
2) A replacement impeller with the necessary reduced diameter may be ordered from the pump manufacturer.
3) In certain cases there may be two separate impeller designs available for the same pump, one of which is of narrower width than that originally furnished. A narrower replacement will have its best efficiency at a lower capacity than the normal width impeller.
Variable speed drives
Oversizing causes the pump to operate to the far left of its BEP on the pump head–capacity curve. Assuming a low static head system, variable speed drives (also known as variable frequency drives) allow the pump to operate near its BEP at any head or flow. In addition, the drive can be programmed to protect the pump from mechanical damage when operating away from the BEP – thereby enhancing mechanical reliability. Furthermore, excessive valve throttling is expensive and not only contributes to higher energy and maintenance costs, but can also significantly impair control loop performance. Employing a throttled control valve, less than 50% open, on the pump discharge may accelerate component wear, thereby slowing valve response.
Variable frequency drives allow pumps to run at slower speeds, with further contributions to pump reliability and significant improvement in mean time between failures (MTBF).
Effect of specific speed
The higher the specific speed selected for a given set of operating conditions, the higher the pump efficiency and therefore the lower the power consumption. Barring other considerations, the tendency should be to favour higher specific speed selection from the point of view of energy conservation.
Clearance
A good wear ring with a proper clearance improves pump reliability and reduces energy consumption. Correct impeller to volute or back plate clearance should also be maintained. Pump efficiency decreases with time because of wear. A well-designed pump usually comes with a diametral clearance of 0.2–0.4%. However, as long as it stays below 0.6–0.8 % its effect on efficiency remains negligible. When the clearance starts to increase beyond these values efficiency begins to drop drastically. For equal operating conditions the rate of wear depends primarily on the design and material of the wear ring. Generally, for noncorrosive liquids, the resistance to wear increases with the hardness of the sealing surface material.
If a pump has a specific speed of 2500, the leakage loss in a new pump will be about 1%. Thus, when the internal clearances have increased to the point that this leakage has doubled, we can regain approximately 1% in power saving by restoring the pump clearance. But if we are dealing with a pump having a specific speed of 750, it will have a leakage loss of about 5%. If the clearances are restored after the pump has worn to the point that its leakage losses have doubled, we can count on 5% power saving.
Change in surface roughness
Depending on the material used in the construction of the pump and the properties of the liquid being pumped, the roughness of the flow path can also change over time. In some instances, the channels may acquire a smooth polish, and in others they may become roughened. Both of these changes can significantly affect pump performance. An increase in casing roughness usually reduces both the total head and the efficiency.
Change in flow path size
The dimensions of the pump's flow path may change with time due to abrasion or erosion – which usually increases the size of the pathways – or due to scale, rust or sedimentation – which usually reduces the size of the pathways. The latter is particularly apt to occur in pumps operating intermittently.
Many installations are provided with two pumps operating in parallel to deliver the required flow under full load. Too often, both pumps are kept online even when demand drops to a point where a single pump can carry the load. The amount of energy wasted in running two pumps at half load when a single pump can meet this condition is significant.
If we want to reduce the flow to half load and still maintain both pumps online, it will be necessary to throttle the pump discharge and create a new system head curve. Under these conditions, each pump will deliver 50% of the rated capacity at 117% rated head, much of which will have to be throttled. Each pump will take 72.5% of its rated power consumption. Thus the total power consumption of two pumps operating under half-load conditions would be 145% of that required if a single pump were to be kept online.
Liquid viscosity
Liquid viscosity affects pump performance. This is because two of the major losses in a centrifugal pump are caused by fluid friction and disc friction. These losses vary with the viscosity of the liquid being pumped, so that both the head capacity output and the mechanical output differ from the original values. As the viscosity of the liquid increases, the head developed by the pump decreases and the efficiency decreases. So in process industries it is essential to maintain good insulation and steam tracing in the suction line of the pump.
Conclusion
Among all the rotating assets in a process plant, centrifugal pumps typically have the best overall potential for energy savings. The different energy-saving methods are:
• Replace throttling valves with speed controls
• Reduce speed for fixed loads
• Install a parallel system for highly variable loads
• Equalize flows using surge vessels
• Replace the pump and/or motor with a more-efficient model.
