to Enhance Pump Performance

Transcription

1 USING SYSTEM CURVES to Enhance Pump Performance Doug Kriebel, P.E. President Kriebel Engineered Equipment Ltd. Editor s Note: This article is the second in a series of Back to Basics that address pump systems. Part 1 appeared in the January/February issue (pp. 9-12). The article is also posted, in its entirety, on our website, A pumping system operates where the pump curve and the system resistance curve intersect. This important fact is one of the most overlooked concepts when engineers design or troubleshoot a pumping system. The pump curve has a head vs. capacity portion, which indicates the change in flow with respect to head at the pump. It is generated by test by the pump manufacturer. The key facts that the pump curve offers were discussed in Part 1. The system resistance curve is the change in flow with respect to head of the system. It must be plotted by the user, and is based upon the conditions of service. These include physical layout, process conditions and fluid characteristics. Anyone using a pumping system must know where the pump is operating. Understanding the system head curve and pump curve relationship is the only way to determine a pump s operating point. SYSTEM HEAD CURVES The common measurement of pressure is psig (pounds per square inch-gauge). Pump curves and system curves are generally shown in pressure units of feet. Reviewing pressure units: absolute gauge psia = psig + barometric pressure (at that location) The conversion from psi to feet of liquid is to divide the pressure, in psi, by its density: Next let s consider the fluid energy requirement. Head is also referred to as the specific energy and is used to calculate total energy: Et = mh, where Et (total energy ft-lbs) = lbsm x ft At any point in a pumping system we consider three components making up its total energy (as expressed in feet of the liquid) Pressure energy (actually PV energy, but volume is considered constant for pumping incompressible fluids). Velocity energy (kinetic). Elevation energy (potential). Heat is ignored since temperature across the pump is usually considered constant, and heat added (or subtracted) to the system is through heat transfer. To derive the conversions to feet: Pressure energy: 8 PUMPS and PROCESSES where is the liquid s density

2 BACK TO BASICS Velocity energy: Elevation: If Hs is the measure of total energy in feet on the suction side of the pump and Hd is the measure of total energy in feet on the discharge side of the pump, the difference in energy levels (H) can be expressed using Bernouilli s equation and adding the friction loss (Hf). H system = Equation 1 (Fig. 1) Where: P1 and P2 are the entrance and exit pressures. hd and hs are the elevations from the pump impeller eye to the entrance and exit liquid levels. vs and vd are the entrance and exit velocities (velocities are given in ft/sec in this article). hfs and hfd are the frictional losses in the suction and discharge pipe system, expressed in feet. The friction loss is the loss in energy due to resistance to flow as the liquid moves through a system. This resistance is due to viscous shear stresses within the liquid and turbulence at the walls of the pipe and fittings. It varies with flow rate, liquid properties (viscosity, solids entrained), size of the pipe, roughness of the pipe s internals, length of travel, and losses at fittings, control valves and the entrance and exits. To simplify Equation 1: Equation 2 This is the total friction loss (at each flow rate) through the system. Equation 3 This is the difference in elevation from the discharge level to suction level. This is the velocity head component of the equation. Equation 4 Velocity head is often ignored since in most cases it is so low compared to other head components. All components of Equation 1 must be consistent and are usually expressed in feet of liquid. Fig. 2. Pumping system For the following calculations, refer to Figure 2. For the pressure component P2-P1 = (5 psig - 0 psig) x 2.31ft/psi x 1.0 = 11. For the static head component h = hd-hs = = 50 Note this is from liquid level to liquid level. The pressure and static head components are constant with flow, and they therefore plot as a straight line. For the velocity head components Assume the tanks are either being filled/emptied at the pumping rate or they are large enough that the downward and upward velocities are very low. If they are the same diameter and the velocity heads cancel, the velocity head component is 0. The friction component, Hf, includes pipe losses and minor losses. There are several ways to calculate the pipe losses: 1. Published Data Sources such as the Crane Handbook, Cameron Hydraulic Data or KSB s Centrifugal Pump Design can be very helpful (see References). 2. The Hazen Williams Formula Where: L = length of pipe in feet v = velocity ft/sec D = pipe diameter in feet C = constant for various pipe materials Equation 5 This is a simple way to go. The disadvantage is that the formula assumes a constant NRe (Reynolds Number) in the MARCH/APRIL

3 BACK TO BASICS turbulent range and a viscosity of 1.13 cs (31.5 SSU), which is water at 60 F. This can yield errors of 20% or more between 32 F and 212 F. 3. The Darcy Formula Equation 6 f, the friction factor, is found based on Reynolds Number (NRe) Equation 7 vis is viscosity in ft 2 /sec Fig. 3. System head curve In laminar flow (NRe = less than 2000) Equation 8 In turbulent flow (NRe greater than 4000), f varies with NRe and the roughness of the pipe expressed as relative roughness = e/d; where e is absolute roughness (in feet) found in references such as Perry s Handbook for Chemical Engineers or the Pump Handbook (see References). f is found in a Moody Friction Factor Chart using NRe and e/d When the NRe is between 2000 and 4000, known as the transition zone, use turbulent flow design parameters. 4. Computerized Flow Simulators You need to know the limits and assumptions upon which the program is based. For complex systems, these simulators may be the only practical method. Fig 4. Two pumps, single destination 5. Spreadsheet Solutions of a Manual Method Because manual solutions are labor intensive, it is useful to set up the equations and solve for various flow rates using spreadsheet analysis. Minor losses include obstructions in the line, changes in direction and changes in size (flow area). The two most common methods of calculating these losses are the equivalent lengths and loss coefficients formulas. Control valve losses are calculated using the Cv for the valve; 10 PUMPS and PROCESSES Equation 9 Note: Hf increases as flow increases and is a parabolic curve. PUTTING THE RESULTS TO USE With the system friction head losses calculated at several flows (the pressure and height components are Fig 5. One pump, two destinations not affected by flow and are considered constant in this case) we can plot the results on Figure 3. We ve assumed a system with design flow of 1200 gpm and calculated friction loss of 27 at 1200 gpm for a hypothetical pipe system. The friction losses at lower flows are also calculated and plotted. The pump s head capacity curve is then plotted on the same graph. Where they intersect is the operating point ( A ). Note the pressure and static head components (61 ) are

4 BACK TO BASICS constant with flow, while the Hf component increases as flow increases. If a control valve in the discharge piping is used, the system head curve could be throttled back to a higher head, therefore lower flow ( B ). The example is as simple as systems get. Often systems may be made up of two or more pumps going to a single destination (Figure 4). This would be typical of a sump pump system with several sump pits going to a common wastewater line, or of pumps feeding different raw materials to a single reactor. There can be one pump, going to two or more destinations (Figure 5). This could be an overhead pump feeding both reflux and product lines. These are solved with the same principles as a simple system, except there are more variables. USING THE NUMBERS From a practical standpoint you will either be designing a new system or you will be examining an existing system to troubleshoot, add capacity, add more destinations, add components or make some other revisions. New Systems If you are designing a new system, the objective is to properly size the pumps by calculating the total head requirements. For Figure 4, there are three possible operating scenarios: both pumps on or both off; pump 1 operating and pump 2 off; pump 1 off and pump 2 on. The first step is to calculate the discharge pressure at each pump for each of the three scenarios at the design flow rates as done with a simple system. This gives the operating pressure (head) range for the two pumps at their design flow rates. Usually flow rates and pipe lengths are determined by the process design. The pipe diameter can then be selected to reduce the design pressure range and simplify sizing of the pump. Often, design ends here and the pumps are selected. However, once designed, all possible variations in pressures, flow rates, and additional operating scenarios must be analyzed to ensure that the actual head ranges do not allow the pumps to operate in a way that limits their reliability. NPSH and System Design Before finishing system design, the NPSHa must be calculated. The NPSHa (Net Positive Suction Head available) is the pressure over the vapor pressure of the pumpage measured at the inlet to the pump. NPSHa is calculated: Equation 10 Where hvp is the vapor pressure of the liquid at its pumping temperature and expressed in feet. The hvp is determined from references. This can be difficult for mixtures. If, at any point, the pressure drops below the vapor pressure of the liquid, the liquid begins to turn to vapor. As the two- phase flow of liquid and vapor passes through the impeller and the pressure increases, the vapor bubbles collapse. This is cavitation. Cavitation is bad and must be avoided. It causes capacity reduction, noise, vibration, pitting of the impeller and eventually failure of a component. Using Figure 6 we will illustrate how varying the conditions can affect the NPSHa. To determine the NPSHa: The suction side is 20 feet of 5 pipe with two elbows and a gate valve. At 300 gpm, the hfs = 1 The pump selected has 23 NPSHr at the design flow of 300 gpm. Example 1 The tank is open to atmosphere, 0 psig, 34 water = P1 ; pumping water at 70 F, hvp =.36 psi =.8 (from references); hs 1 = 10 then (from Equation 10) The NPSHa = P1 + hs - hvp - hfs = = 42.2 Since the NPSHa is 42.2, at 300 gpm and the NPSHr = 23. The pump has adequate NPSHa. Example 2 All conditions remain the same except the tank is drawn down to level hs 2 = -10 The NPSHa is now calculated as = The NPSHa is less than the NPSHr of 23 and the pump cavitates. Example 3 All conditions remain the same as Example 1 except the water is now 190 F; hvp = 9.34 psi = 22.3 The NPSHa is now = 22.7 and the pump cavitates. Example 4 Conditions are the same as any above, except the suction pipe diameter is 3 instead of 5. The friction loss is then hfs = 10 (instead of 1 ). All the above NPSHa results would be reduced by 9. The lessons to be learned on the suction side are to have as REFERENCES 1. Cameron Hydraulic Data, Ingersoll-Dresser Pump Company, Crane Co., Technical Paper 410. Flow Through Valves, Fittings and Pipe. Crane Co., Karassik, Igor, et al. Pump Handbook, McGraw-Hill, Holzenberger et al, KSB Centrifugal Pump Lexicon, KSB, Perry, et al. Perry s Handbook for Chemical Engineers, McGraw-Hill, 1984 MARCH/APRIL

5 BACK TO BASICS much static suction head as possible (hs); locate the pump as close as possible to the suction vessel, with a minimum suction side frictional pressure drop (hfs); and be careful of liquids near their vapor pressure (hvp). WHAT CAN GO WRONG? A centrifugal pump will run where the system head curve tells it to run. The point where the centrifugal pump performance curve and system head curve intersect is the operating point. It s important to understand where the operating point is at all times and to avoid the dangerous areas of pump operation. PUMP DANGER ZONES Horsepower A pump must always have a driver with more HP than the pump consumes. With most centrifugal pumps, as the pump flow increases, the BHP increases to a maximum, and there must always be enough HP available from the driver to supply the needs of the pump. This means that as TDH falls, the flow goes up, and so does the BHP. Under-sizing overloads the driver. NPSH As the pump flow increases, the NPSHr increases. As flow increases, NPSHa decreases (hfs increases with flow) and there must always be more NPSHa than NPSHr or the pump cavitates. No Flow It is possible for the TDH to rise to a 12 PUMPS and PROCESSES Fig 6. NPSHa point where there is no flow. This is called shut off and means no flow is going through the pump. At this point, the inefficiencies of the pump will cause the temperature in the pump to rise, thereby increasing the temperature of the pumpage. This will eventually cause the pumped liquid to vaporize and the close clearances such as ring fits and mechanical seal faces will not receive lubrication. Eventually the seals fail or the pump seizes. High temperature also causes the pump to distort and change clearances. Failure of some component will occur. Low Flow It is possible for the TDH to rise to a point of low flow where damage other than no flow occurs. Such things include: Low flow temperature rise As the flow decreases, the efficiency falls and there is more BHP needed to drive the pump than is needed to move the liquid through the TDH. The excess BHP goes into the pump as heat. The flow is lower (less to carry away the heat) and therefore the temperature rise is greater as flow drops. It is possible to reach a point where the temperature rises to the point it reduces the NPSHa below the NPSHr and flashing occurs as above. This point is the thermal minimum flow and is a function of not only the pump but the system characteristics as well. Low flow recirculation/separation damage As flow decreases, the liquid begins to recirculate in both the impeller eye and impeller discharge (at the volute cutwater or diffuser). This recirculation will cause separation, eddy currents and turbulence, which will cause the same noise, vibration and damage as cavitation (Fig. 7). The minimum flow required to prevent low flow recirculation/separation damage is called the continuous minimum flow. It is specified by the pump manufacturer based on the impeller eye geometry and Nss. Radial or axial thrust damage All pumps use bearings and balancing devices to prevent the rotating parts from contacting the stationary parts. These are pressure and flow related and they may limit the flow of the pump to a minimum or maximum. The pump manufacturer must advise the limits based on the pump design. NEXTTIME: How systems negatively affect pump performance. Throughout his 30-year career, Doug Kriebel has presented many training and instructional programs. He has held positions with a major pump manufacturer, as well as in both the electric utility and chemical equipment industries. Kriebel holds a BSChE, is a registered Professional Engineer and is a member of AIChE. Fig 7. Low flow recirculation p p