Pump Analysis 1) Capacity - volume of liquid pumped per unit of time 2) Head

Transcription

1 Pump Analysis 1) Capacity - volume of liquid pumped per unit of time 2) Head - several terms are used as shown in the attached Figures Static Suction Head, h s The difference in elevation between the suction side liquid level and the centreline of the pump impeller. - if the suction side liquid level is below the centreline of the pump impeller it is a "static suction lift" - waste water pumps are generally installed with a small static suction head to avoid the requirement for pump priming Static Discharge Head, hd The difference in elevation between the discharge side liquid level and the centreline of the pump. - if the pipe discharges to atmosphere the discharge liquid level is the elevation of the pipe outlet. Total Static Head The difference between the static discharge head and the suction head i.e. hd -hs note: "static suction lift" is a -ve static suction head.

2

3 Total Dynamic Head or Pump Head The head against which the pump must work when water or waste water is being pumped. V 2 d H pump = h d + 2g + h fd + h md - h s - ent. loss - h fs - h ms pipe friction losses in the suction and discharge pipes h fd, h fs given by flv2 D2g Q 1.85 or L 0.278CD 2.63 fitting losses in the suction and discharge pipes h md, h ms given by K V2 2g V d 2 2g velocity head in the discharge pipe which is lost upon exit ent. loss head loss upon entrance to the suction inlet, similar to a fitting loss, usually expressed as proportional to the velocity head H pump = h d - h s + ent. loss + h fs + h fd + h ms + h md + V d 2 2g Hpump can also be determined by applying Bernoulli's equation between the suction and discharge nozzles.

4 3) Pump efficiency and power input Pump efficiency - ratio of useful output power of the pump to the input power to the pump E p = pump output P i = γ QHpump (SI units) = P i γqh pump bhp x 550 (English) Ep is the efficiency Pi is the power input ( kw, kn m/s) γ is the specific weight of the water (kn/m3) Q is the capacity (m3/s) Hpump is the total dynamic head (m) The maximum efficiency is generally in the range of 60 to 80% Pump Performance Curves - pump performance is measured with a pump test - Hpump is measured for various flowrates by throttling a valve on the discharge line (pump running at a constant rpm) - the pump input power and efficiency are measured at the same time - the plots of these variables versus discharge are known as the pump characteristic curves - head-capacity curve - power curve - efficiency curve

5 - the point of highest efficiency is called the best efficiency point or "bep" - pump characteristic curves are provided by the manufacturer System-Head-Capacity Curve - a plot of the total static head and the headlosses in a system for the range of capacities from 0 to the maximum expected discharge - plot the pump head-capacity curve on the same axes as the system-head-capacity curve - the intersection indicates the head and the capacity the pump would operate at for the given piping system - the intersection point is called the "operating point" Pump Operating Range - ideally pumps should be operated at their "bep" - at the bep radial loads on the pump bearings are at a minimum - radial loads increase towards the "shutoff" or "runout" ends of the curve - towards runout "cavitation" is a potential problem - cavitation occurs when the absolute pressure at the pump inlet falls below the vapour pressure of the liquid and vapour bubbles form - vapour bubbles are carried into the pump and encounter higher pressure, they then abruptly collapse causing a hammering action on the pump interior surface - the hammering will pit and damage the impeller surface

6

7 - manufacturers provide data on a recommended NPSH (net positive suction head) required to prevent cavitation - the NPSH available is: NPSH a = h s - h fs - V s 2 2g - h ms + P atm γ - P vapour γ NPSHa > NPSHrecommended to prevent cavitation - toward shutoff recirculation of liquid within the pump causes vibration and hydraulic losses - the best practice is to operate the pump between 0.6Qbep and 1.2Qbep - the 'operating envelope' can be plotted for various impeller sizes and operating speeds

8 Multiple Pump Operation Parallel Operation When two or more pumps operate in parallel the capacities are additive neglecting station friction losses. An evaluation of station friction losses must be incorporated to accurately predict the system operating point, and the individual operating capacities and heads. The following procedure is recommended: 1. Omit the suction and discharge friction losses within the pumping station (station losses) when developing the system head-capacity curve. 2. Derive a modified head-capacity curve for each pump by subtracting the appropriate station losses from the standard head-capacity curve. 3. Add the modified pump capacities at a given head to obtain the combined modified pump head-capacity curve 4. The point of intersection of the combined modified pump head-capacity curve and the system head-capacity curve is the operating point for the pumps in parallel. 5. The capacity at which each individual pump is operating is given by the intersection of their modified head-capacity curve and the system pumping head.

9 6. The actual operating head of each pump is slightly higher to overcome the station friction losses. The operating head of each pump is determined using its original head-capacity curve and the operating capacity determined in Step 5. Series Operation When two or more pumps operate in series the heads are additive. An evaluation of friction losses specific to series operation must be incorporated to accurately predict the system operating point, and the individual operating capacities and heads. The following procedure is recommended: 1. Derive a system head-capacity curve for the piping system for series operation. 2. Plot the individual pump system-head capacity curves. 3. Add the individual pump heads at given capacities to obtain the combined pump head-capacity curve 4. The point of intersection of the combined pump head-capacity curve and the series operation system head-capacity curve is the operating point for the pumps in series. 5. The operating head of each pump is given by the intersection of the individual head-capacity curves with the operating capacity determined in Step 4.

10

11 Pump 1 operates, losses through 2 bends, 2 valves, 2 tees Pump 2 Pump 1 Pump 2 operates, losses through 2 bends, 2 valves, 2 tees Pump 2 Pump 1 Pump 1 and 2 operate, losses through 6 bends, 3 valves Pump 2 Pump 1 Pump 1 and 2 in series H Pump 2 System losses through Pump 1 and 2 System losses through Pump 2 Pump 1 Q