The short answer
High Service Pumps
High service pumps are usually horizontal split case or vertical turbine centrifugal pumps taking suction from the clearwell and discharging into the distribution system, and they operate where their curve crosses the system curve, which moves as demand and tank levels change. Each pump is most efficient near its best efficiency point, and running far from it wastes energy and shortens life. A drive changes the curve according to the affinity laws, which is valuable where the system head is mostly friction and of limited value where it is mostly static, since below a certain speed the pump cannot overcome the static head at all. Pumps in parallel add flow at the same head, and dissimilar pumps have to be checked for the one that gets pushed off its curve. Pump control valves, check valves, isolation, and protection on vibration, temperature, seal water, and suction pressure complete the installation, and wire-to-water efficiency computed from flow, head, and power is the measure of pump condition.
Key points
- The operating point is where the pump curve meets the system curve; both move, and the pump has to live everywhere they meet.
- Best efficiency point is the design target; a pump run far left or right of it vibrates, heats, and wears.
- Affinity laws: flow with speed, head with speed squared, power with speed cubed. Savings from a drive depend on how much of the head is friction.
- Static head sets a minimum useful speed; below it the pump delivers nothing and heats the water.
- Parallel pumps add flow at equal head; a smaller pump in parallel with a larger one can be forced to shutoff.
- Wire-to-water efficiency from flow, head, and power, trended per pump, is the wear indicator.
Types
| Type | Arrangement | Where it fits |
|---|---|---|
| Horizontal split case | Double suction impeller, motor beside the pump, flooded suction from the clearwell | The common high service pump; easy to maintain, efficient in large sizes |
| Vertical turbine | Bowl assembly in a can or in the clearwell, motor above | Where suction lift or footprint rules; the clearwell can be the sump |
| End suction | Single suction, close coupled or frame mounted | Smaller stations and booster duty |
| Multistage | Several impellers in series | High head at moderate flow, such as feeding a high zone |
Curves
The pump curve, from the manufacturer, gives head against flow at a fixed speed, with efficiency, power, and required net positive suction head drawn on the same axes. The system curve gives the head the system demands at each flow: a static part, which is the elevation difference between the clearwell and the tank or the pressure the system must be held at, and a friction part that rises with the square of the flow. The pump operates where the two curves cross. When a tank fills, the static head rises and the crossing moves left to lower flow; when demand rises and the tank falls, it moves right. When a second pump starts, the system curve is the same but the combined pump curve adds the flows at each head, and each pump moves left on its own curve.
Best efficiency point
Every pump has one flow at which its efficiency peaks, and the manufacturer draws a preferred operating range around it, commonly from about 70 to 120 percent of that flow. Inside the range the pump is smooth. To the left, at low flow and high head, recirculation inside the impeller causes vibration, heating, and bearing and seal wear, and at shutoff the water in the casing heats rapidly. To the right, at high flow and low head, the required suction head rises and the pump cavitates. A pump selected for the system at its design point will spend hours at the edges of the range as tanks and demand move; the selection is checked across the whole range of system curves, not at one point.
Affinity laws and drives
Q₂/Q₁ = N₂/N₁ H₂/H₁ = (N₂/N₁)² P₂/P₁ = (N₂/N₁)³
- Q = flow, H = head, P = power, N = speed
- The relations hold for the pump curve moving with speed; the system decides where on the new curve the pump runs
The power law is where drive savings come from: a pump slowed to 80 percent speed draws roughly half the power. But the flow it delivers depends on the system curve. Where the head is mostly friction, as in a long transmission main, slowing the pump reduces flow and head together along a curve that follows the system, and the saving is close to the affinity prediction. Where the head is mostly static, as in pumping into an elevated tank, the pump must produce the static head at any flow, and below the speed at which its shutoff head equals the static head it delivers nothing at all. Between that minimum speed and full speed the flow changes quickly and the saving is modest. The economic case for a drive on a high service pump depends on that ratio, and a system curve on the pump curve shows it in one glance.
Water horsepower = Q × H / 3960 Wire-to-water efficiency = Water horsepower / (Input kW × 1.341)
- Q = flow in gallons per minute
- H = total head in feet, from suction and discharge pressure corrected for elevation and velocity
- Input kW = electrical power to the drive or the motor
Parallel operation
Pumps in parallel deliver the sum of their flows at a common head. Identical pumps share the flow; each runs at the same point on its own curve. A smaller pump in parallel with a larger one delivers flow only when the system head is below its shutoff head; as the larger pump raises the head, the smaller one is pushed toward shutoff, where it heats and does nothing useful. The check on any parallel combination is to draw the combined curve against the system curve and confirm that every pump in the combination is within its range at every system condition. A drive pump in parallel with a constant speed pump has the same problem in reverse: at low speed the drive pump cannot reach the head the fixed pump produces, and it runs at shutoff until its speed rises.
Valves and fittings
- Isolation valves on suction and discharge for maintenance.
- A check valve on the discharge, of a type chosen for the surge behavior of the main: a slow-closing swing check slams on a long main; a silent check or a controlled-closing valve does not.
- A pump control valve where surge requires it: opens after the pump starts, closes before it stops, on a controlled timing, with limit switches to the controller.
- A suction strainer where the clearwell can carry debris, with a differential alarm.
- Air release at the high point of the pump casing and at the discharge where air can collect.
- Pressure gauges and transmitters on suction and discharge, and a flowmeter on the station discharge or per pump.
Protection
| Condition | Detection | Response |
|---|---|---|
| Low suction pressure or clearwell level | Transmitter, with a delay | Stop; protects against cavitation and dry running |
| High discharge pressure | Transmitter | Stop or reduce speed; a closed valve downstream |
| Fail to prove flow | Flowmeter or flow switch, after a start delay | Stop and alarm; start the next pump |
| High vibration | Vibration switch or transmitter on the bearings | Alarm, then stop at the trip level |
| High bearing or winding temperature | RTDs | Alarm, then stop |
| Seal water loss on packed pumps | Flow switch | Alarm, stop after a delay |
| Motor overload, phase loss | Overload relay, monitor relay | Trip in hardware |
| Excessive starts | Counter in the controller | Block the next start until the interval has elapsed |
Watching the pump wear
A worn impeller or wear ring delivers less head at a given flow and speed and uses more power for the water it moves. The control system can see it: compute the water horsepower from the flow and the head, divide by the input power, and trend the efficiency for each pump at comparable operating points. A pump whose efficiency has fallen by several points since commissioning is due for inspection, and a station that trends this schedules pump overhauls on evidence rather than on failure. The same data shows a pump running far from its best efficiency point most of the time, which is a selection or a staging problem worth fixing.
Frequently asked questions
- Will a drive on the high service pump save energy?
- It depends on the head. On a system where most of the head is friction, a drive saves a great deal by matching flow to demand at low speed. On a system pumping into a tank with a high static head, the pump has to run near full speed to deliver anything and the drive saves little beyond the soft start. Draw the system curve and decide from it.
- How low can a drive pump run?
- Down to the speed at which its shutoff head equals the static head, plus a margin so it actually delivers flow, and in any case above the minimum the motor and pump need for cooling and lubrication. On a high static head system that minimum can be 80 or 90 percent of full speed; the drive is then a soft starter with a narrow control range.
- Why does the small pump in the station never seem to move water when the big one runs?
- Because the big pump raises the system head above the small pump shutoff head, and the small pump is pushed off its curve. That combination should not run; the staging logic should either run the small pump alone or the big one alone, or the small pump needs to be replaced with a match.
- What total head should I use for the efficiency calculation?
- Discharge pressure minus suction pressure, converted to feet, corrected for the elevation difference between the two gauges and, where the suction and discharge pipes are different sizes, the difference in velocity head. Use the same taps every time so the trend is consistent.
Related topics
- High Service PumpingPumping finished water into distribution: the control modes of tank level, pressure, and flow, how they interact with the clearwell and contact time, staging and surge, the telemetry the station depends on and its fallback, energy scheduling, and the alarms.
- Distribution Pressure ControlHow a booster or high service pump station holds discharge pressure with variable speed pumps, how pumps are staged, and what stops the system hunting.
- Booster Pump StationsInline stations that raise pressure from one zone to another: suction pressure protection, discharge pressure control with drives, staging, the hydropneumatic tank alternative, surge and check valve behavior, and the interlocks that keep a booster from collapsing the zone it draws from.
- Variable Frequency Drives in Pump PanelsWhat a drive adds to a pump panel and what it costs: speed control, soft start, reduced inrush, and diagnostics against heat, harmonics, cable and motor stress, bypass, and the minimum speed below which a pump does no work.
- Differential PressureThe differential pressure transmitter as an instrument: what it measures, static and overrange ratings, the three-valve manifold and the order of operations that protects the cell, and the applications from filter head loss to membrane TMP, flow, and level.
- Pump Runs But No FlowThe motor turns and nothing moves. How to tell a clogged impeller from an air-bound pump, a closed valve, a stuck check, a broken coupling, or reverse rotation, using motor current and discharge pressure before pulling anything.
Direct contact
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