What affects metering pump performance?
Hydraulically balanced reciprocating metering pumps depend on the whole system around them. Designers have to weigh fluid properties (vapor pressure, specific gravity and viscosity), suction pressure at the inlet, backpressure at the outlet, pipe length and diameter, and freedom from air and foreign matter. Success comes from considering all of them together.
This guide uses PULSAfeeder hydraulic diaphragm pumps as the example, but the principles apply to reciprocating metering pumps in general.
Which conditions make a metering pump installation difficult?
Most metering pump problems trace back to a short list of system conditions. If your installation has one or more of them, give the suction and discharge hydraulics extra attention before buying equipment. The more items that apply, the more important a full calculation becomes.
- Piping with excessive fittings, valves and bends
- Suction lift or low suction head
- Undersized suction or discharge piping
- Dissolved gases in the product, or unusually high vapor pressure
- Pumping near the net positive suction head required (NPSHR)
- Unusually high product specific gravity
- High product viscosity
- Long suction or discharge lines
- Pumping downhill (suction pressure higher than discharge pressure)
- Pumping at unusually high or low temperatures
- Solids content or slurry, where pipe size matters to minimize settling
- Outdoor installation in a cold climate
- Changing elevations in suction or discharge lines, where air traps cause problems
Why are metering pumps harder to pipe than centrifugal pumps?
Centrifugal and rotary pumps produce steady flow with little change in fluid velocity. A reciprocating metering pump starts and stops the liquid every stroke, so both suction and discharge lines see peak velocities and accelerations. The force needed to start the liquid moving on each stroke is significant, and the piping must allow for it.
Over one revolution of the drive cam:
- At 0°, the fluid is at rest.
- On the discharge stroke, the fluid accelerates to peak velocity at 90°.
- The suction stroke begins with the fluid at rest at 180° and reaches peak velocity at 270°.
What should metering pump piping include?
Good metering pump piping lets the liquid move freely, can be taken apart for service, is well supported and is chemically compatible. Use full port shutoff valves, unions or flanges at the pump, a suction strainer for clean liquids and proper pipe supports. Do not weld pipe nipples or pipe to valve housings.
- Shutoff valves on the suction and discharge lines that open to the full pipe diameter. Ball valves are preferred. Regulating valves (needle, diaphragm, globe and gate) are not recommended.
- Unions or flanges on suction and discharge piping.
- An inlet strainer, if the product is not a slurry.
- Hangers and straps to support the piping.
- Materials unaffected by product corrosion or galvanic corrosion.
How much suction head does a metering pump need?
Every reciprocating metering pump has an NPSHR: the pressure above the fluid's absolute vapor pressure that must exist at the suction port throughout the entire stroke. It prevents cavitation in the reagent head and is essential for metering accuracy. NPSHA must exceed NPSHR, and NPSHA plus vapor pressure must also exceed a minimum suction head.
NPSHA for a metering pump is atmospheric (or tank) pressure plus the pressure from the liquid head above the pump, minus vapor pressure, minus friction losses, minus the acceleration losses described above. The manufacturer's equations differ for liquids below and above 50 centipoise. If suction pipe sizes vary along the line, calculate the acceleration and deceleration losses separately for each diameter section, changing only length and diameter, and add them.
The manufacturer also lists a minimum absolute pressure, the minimum suction head (MSH). NPSHA plus vapor pressure (PV) must exceed this value to keep the pump's hydraulic system stable.
How much backpressure does a metering pump need?
System backpressure must be at least 5 psi (0.35 bar) higher than suction pressure, and must not exceed the pump's rated discharge pressure. Without that difference, liquid can flow straight through the pump from the higher pressure suction to the lower pressure discharge, which is called flow through.
Flow through happens when pumping downhill. It can cause pump failure and unwanted flow when the pump is shut down. If backpressure is not at least 5 psi (0.35 bar) above suction pressure, install a backpressure valve in the discharge piping. Total backpressure includes system pressure, static head, friction and acceleration losses, with separate equations for liquids below and above 50 centipoise.
How do you size metering pump piping?
Size piping on the peak fluid velocity, not the average velocity. Because a reciprocating pump only moves liquid during part of each stroke, the instantaneous peak flow is far higher than the average. For a simplex pump with sinusoidal motion, peak flow is about π (3.14) times the average flow.
What does a pulsation dampener do on a metering pump?
A pulsation dampener (such as PULSAtrol) is a pneumatically charged diaphragm in a chamber that stores energy from the accelerating fluid and releases it as flow slows. On the suction side it improves suction conditions; on the discharge side it protects equipment and smooths flow. Mount it as close to the pump as possible.
- Suction side: reduces pressure losses from acceleration and improves suction conditions.
- Discharge side: protects sensitive equipment from pressure spikes and converts pulsating flow to nearly linear flow by reducing peak flow and pressure.
- Smaller piping: with dampeners on both lines, many installations can use smaller, less expensive pipe.
- Smaller pump: a smaller, faster stroking pump can replace a larger, slower, more costly one.
With a dampener installed, the pipe length used in the acceleration calculations becomes only the distance from the pump to the dampener. The rest of the line is treated with standard friction losses from the Darcy formula (or standard friction loss tables), subtracted from NPSHA or added to backpressure. See our pulsation dampeners.
Is 4 feet of suction head enough? A worked example
For 93 percent sulfuric acid at 240 GPH on a PULSA Series pump, 4 ft (1.22 m) of flooded suction head is enough. NPSHA comes to about 9.9 psi, well above the 5 psi NPSHR, and NPSHA plus vapor pressure is 9.91 psia, above the 9.5 psia minimum suction head.
| Condition | Value |
|---|---|
| Fluid | 66° Bé sulfuric acid (93%) |
| Specific gravity | 1.83 |
| Viscosity | 25 cP |
| Vapor pressure | 0.01 psia (0 bar(a)) |
| Flow | 240 GPH (908.4 LPH) |
| System pressure | 100 psig (6.9 bar) |
| Location | Sea level, supply tank open to atmosphere |
| Suction piping | 20 ft (6.1 m) of 1.61 in (40.9 mm) ID pipe |
| Pump | PULSA Series, 58 strokes per minute |
| Suction head available | 4 ft (1.22 m) |
- NPSHR for a PULSA Series pump is 5 psi (0.35 bar).
- Viscosity is below 50 cP, so the low viscosity NPSHA equation applies.
- Suction head pressure: PH = 4 ft × 0.433 psi/ft × 1.83 = 3.17 psi. In metric: PH = 1.22 m × (1 bar / 10.21 m H₂O) × 1.83 = 0.22 bar.
- Atmospheric pressure at sea level is 14.7 psia (1.01 bar).
- After friction and acceleration losses, NPSHA exceeds NPSHR, so there is no cavitation in the reagent head.
- NPSHA + PV = 9.91 psia (0.68 bar), which exceeds the 9.5 psia (0.66 bar) minimum suction head for a PULSA Series pump, so the hydraulic system will be stable.
Conclusion: 4 ft of suction head is sufficient for this installation.
PCT represents Pulsafeeder and can run these calculations for your application. Request a quote or see our metering pumps.