Discharge Piping, Friction Losses, Hose and Routing

D/G-04 Installation

Friction Losses

Calculating Friction Losses in Suction Piping

When following the above recommendations (under “inlet Piping”) for minimum hose/pipe I.D. and maximum length, frictional losses in the suction piping are negligible (i.e., Hf = 0) if you are pumping a water-like fluid.

When pumping more-viscous fluids such as lubricating oils, sealants, adhesives, syrups, varnishes, etc., frictional losses in the suction piping may become significant. As Hf increases, the available NPSH (NPSHa) will decrease, and cavitation will occur.

In general, frictional losses increase with increasing viscosity, increasing suction-line length, increasing pump flowrate, and decreasing suction-line diameter. Changes in suction-line diameter have the greatest impact on frictional losses: a 25% increase in suction-line diameter cuts losses by more than two times, and a 50% increase cuts losses by a factor of five times.

Consult the factory before pumping viscous fluids.

Minimizing Acceleration Head and Frictional Losses

To minimize the acceleration head and frictional losses:

•Keep inlet lines less than 3 ft (1 m) long

•Use at least 5/8 in. (16 mm) I.D. inlet hose

•Use soft hose (low-pressure hose, noncollapsing) for the inlet lines

•Minimize fittings (elbows, valves, tees, etc.)

•Use a suction stabilizer on the inlet.

Net Positive Suction Head

NPSHa must be equal to or greater than NPSHr. If not, the pressure in the pump inlet will be lower than the vapor pressure of the fluid— and cavitation will occur.

Calculating the NPSHa

Use the following formula to calculate the NPSHa: NPSHa = Pt + Hz - Hf - Ha - Pvp

where:

Pt = Atmospheric pressure

Hz = Vertical distance from surface liquid to pump centerline (if liquid is below pump centerline, the Hz is negative)

Hf = Friction losses in suction piping

Ha = Acceleration head at pump suction

Pvp = Absolute vapor pressure of liquid at pumping temperature

NOTES:

•In good practice, NPSHa should be 2 ft greater than NPSHr

•All values must be expressed in feet of liquid

Atmospheric Pressure at Various Altitudes

Altitude	Pressure	Altitude	Pressure
(ft)	(ft of H2O)	(ft)	(ft of H2O)
0	33.9	1500	32.1
500	33.3	2000	31.5
1000	32.8	5000	28.2

Discharge Piping

NOTE: Consult the Factory before manifolding two or more pumps together.

NOTE: Single-acting pumps create a pulsing flow. Using pulsation dampening devices in the discharge line can reduce or eliminate this.

Hose and Routing

Use the shortest, most-direct route for the discharge line.

Select pipe or hose with a working pressure rating of at least

1.5times the maximum system pressure. EXAMPLE: Select a 3000-psi W.P.-rated hose for systems to be operated at 2000- psi-gauge pressure.

Use about 6 ft (1.8 m) of flexible hose between the pump and rigid piping.

Support the pump and piping independently.

Pressure Regulation

IInstall a pressure regulator or unloader in the discharge line. Bypass pressure must not exceed the pressure limit of the pump.

Size the regulator so that, when fully open, it will be large enough to relieve the full capacity of the pump without overpressurizing the system.

Locate the valve as close to the pump as possible and ahead of any other valves.

Adjust the pressure regulating valve to no more than 10% over the maximum working pressure of the system. Do not exceed the manufacturer’s pressure rating for the pump or regulator.

Route the bypass line to the supply tank, or to the suction line as far as possible from the pump (to reduce the chance of turbulence and cavitation).

If the pump may be run for a long time with the discharge closed and fluid bypassing, install a thermal protector in the bypass line (to prevent severe temperature buildup in the bypassed fluid).

CAUTION: Never install shutoff valves in the bypass line or between the pump and pressure regulator or relief valve.

Provide for permanent or temporary installation of a pressure gauge to monitor the discharge pressure at the pump.

For additional system protection, install a “pop-off” safety relief valve in the discharge line, downstream from the pressure regulator.

7	D04-991-2400 5/1/04