Improving Pump Efficiency with DuPont™ Vespel® CR-6100
According to a major centrifugal pump OEM, energy consumption accounts for 44% of the life cycle cost of a centrifugal pump. Improving pump efficiency by using Vespel® CR-6100 can make a significant contribution to reducing this cost.
The efficiency improvement from Vespel® CR-6100 results from reduced clearance at the internal wear components in the pump: pump wear rings, inter-stage rings, center-stage bushing, and throttle bushing. These components form the barriers between high-pressure and low-pressure areas within the pump. Because Vespel® CR-6100 is a non-seizing material, the clearance at these interfaces can be reduced, reducing internal leakage losses from high-pressure to low-pressure areas, and thereby increasing efficiency.
There are 4 factors which determine how much the efficiency will increase, and a final factor which will determine the long term savings of the upgrade.
Factor 1: Pump Specific Speed
Look up “specific speed” in any pump design textbook and you will find a diagram showing pump impellers ranging from radial flow impellers on one end to axial flow impellers at the opposite end. Radial flow pumps have a low specific speed; axial flow pumps have a high specific speed.
Efficiency gains from reducing clearance are directly related to the pump specific speed. Because low specific speed pumps produce more head relative to the flow rate, internal leakage past the wear rings has a much larger impact on efficiency. Conversely high specific speed pumps produce limited head relative to the flow rate and internal leakage past the wear rings has a minor impact on overall pump efficiency.
Factor 2: Pump configuration
Wear rings are only one of the internal components controlling internal leakage from high-pressure to low-pressure areas. The chart shown in Figure 1 is applicable for a single-stage pump. Multi-stage pumps have inter-stage rings, throttle bushings, and center-stage bushings which also contribute to the efficiency gain. Reducing the clearance at all of these components will also increase the pump efficiency.
Factor 3: % reduction in clearance
The rule of thumb for new pumps is that the wear ring clearance using Vespel® CR-6100 should be 50% of the API value for metal components. However, many pumps operate with clearance significantly higher than API clearance. If the wear ring clearance can be reduced by 75% instead of 50%, the efficiency gain from the upgrade will roughly double.
Factor 4: Cavitation
One of the hidden benefits of Vespel® CR-6100 is that reducing clearance also helps to suppress pump cavitation. The reason is that the internal leakage past the wear rings enters directly into the impeller eye, disrupting the flow, increasing the fluid temperature, and increasing the pump NPSHR. This is why when pump wear rings are worn and running with excessive clearance a pump is very likely to cavitate. Depending on the fluid characteristics and degree of cavitation, suppressing the cavitation with reduced clearance can also improve pump efficiency.
Life Cycle Factor: Wear Rate
Many older pumps and some new pumps use basic materials like bronze and cast iron for the pump wear rings. These materials are used because they are relatively non-seizing and inexpensive; however, they come with a very high price in the long term. The metal-to-metal friction between rotating and stationary parts can cause very high wear rates in bronze or cast iron, leading to significant efficiency losses within just a couple of years of operation.
In one set of field data which was shared with Boulden, several multi-stage pumps fitted with cast iron or bronze wear rings lost an average of 5% efficiency within only 4 years. When those pumps were converted to Vespel® CR-6100 case rings, the efficiency loss over time was only about 2% over 7 years—in line with the performance of premium, hardened stainless alloys.
Putting it all together
Field tests and test lab data confirm that reducing wear ring clearance produces a significant increase in pump efficiency.Putting all the information together to identify the pumps with a high potential for energy savings, the following table should prove helpful.
Table 1: Pump Efficiency Gain Potential
|Pump Type||50% Clearance Reduction||67% Clearance Reduction||75% Clearance Reduction|
|Multi-stage horizontal pump or radial flow pump||4-6%||6-9%||8-12%|
|Typical Process Pump||2-4%||3-6%||4-8%|
+2% or more if cavitation is suppressed
+3-4% in future years if replacing bronze or cast iron