Glossary of Pump Terms: P
Packing:
The soft rings that mechanical seal replace to stop leakage. Packing must leak because it works on the theory of a series of pressure drops to reduce the stuffing box pressure to the point where the leakage is acceptable. A minimum of five rings of packing is required to do this.
Parallel operation:
The pumps are discharging to a common header. It is important that the impeller speed and outside diameters are the same or one of the pumps will throttle the other.
Pascal:
A metric unit for pressure. 100 kPa = one atmosphere.
pH:
A measure of the acidity or the alkalinity of a fluid. The scale ranges from 0 (acid) to 14 (alkali) with 7 considered neutral.
Pipe friction loss:
The positive head loss due to friction resistance between the pipe walls and the moving liquid.
Pipe strain:
The strain on the pump volute caused by the piping. It will cause excessive mechanical seal movement and can cause contact between rotating and stationary pump and seal components.
Pitting:
Surface voids caused by corrosion, erosion or cavitation.
Power end:
The end of the pump that attaches to the power source and does not get wet from the pumpage. The bearings are in this part.
Power factor:
A measure of how the voltage leads or lags the amperage.
Precision bearing:
Ball or roller bearing as opposed to a sleeve or babbitt bearing.
Pressure drop:
Referring to the loss of pressure from the outside to the inside of the mechanical seal faces or across the individual rings of packing.
Pressure gradient:
The pressure drop between the seal faces. Usually illustrated by a wedge.
Pressure head:
The pump head exerted by atmospheric pressure or any additional pressure that might be in the vessel.
Prime:
A charge of liquid required beginning the pumping action of centrifugal pumps when the liquid source is
lower than the pump.
Pump out vane:
Located behind the impeller shroud in some impeller designs to lower stuffing box pressure. Should no be used in hot well condensate pumps or any pump running with a negative stuffing box pressure.
Pumping ring:
Used with a convection system to get circulation between two mechanical seals. Absolutely necessary if oil is used as a barrier fluid because of oil’s poor specific heat.
Pusher seal:
A design that has a spring loaded dynamic elastomer or rubber like part. A very poor design that should be avoided.
Glossary of Pump Terms: Q
Quench:
The introduction of a fluid outside the seal to cool the product, or dilute any leakage across the seal faces.
Quill shaft:
A hollow shaft with another shaft inside it.
Glossary of Pump Terms: R
Radial:
90 degrees to the centerline of the shaft.
Radial bearing:
In an end suction centrifugal pump it is the bearing located closest to the stuffing box. This bearing handles most of the radial loads put on the impeller.
Renkin scale:
Used to measure low temperatures in the Fahrenheit scale. Similar to Kelvin which is used to measure low temperatures in the Centigrade system.
Repeller:
A second impeller used to lower the stuffing box pressure.
Rigid shaft:
Shafts with a rotating speed lower than its first critical speed.
RMS:
Root mean square. A measure of surface finish or smoothness. Metric uses C.L.A. or centerline average for the same purpose.
Rockwell “C”:
The scale most often used to measure hardness of the hard seal face.
Rotating seal:
When the spring loaded or moveable portion of the seal rotates with the shaft.
Run out:
Twice the distance that the center of the shaft is displaced from the axis of rotation.
Glossary of Pump Terms: S
Seal faces:
The lapped faces that provide the primary seal in a mechanical seal.
Seal life:
Seals should run leak free until the sacrificial face (usually carbon/ graphite) is worn away. More than 85% of the mechanical seals in use today fail prematurely.
Seal only pump:
The pump does not have a conventional stuffing box so it could not be supplied with soft packing.
Self align:
A method of keeping both mechanical seal faces square to the rotating shaft.
Series operation:
The pumps are connected with the discharge of the first pump discharging to the suction of the other. The speed and impeller widths must be the same or the difference in capacities could cause cavitation or over heating problems.
Shaft packing:
The soft packing supplied by pump manufacturers. Mechanical seals are replacing most of these leaking packing.
Shelf life:
Usually refers to the seal elastomer. Buna “N” is the biggest problem because of Ozone attack.
Shut off head:
The maximum head that the pump can generate with a given impeller outside diameter and horsepower driver.
Sintered material:
Formed from a powder as opposed to being melted and poured into a mold.
Sleeve bearing:
A non-precision bearing. Usually manufactured from carbon, Teflon. brass etc. Allows too much axial and radial movement for most mechanical seal applications.
Slip stick:
An alternating slipping and sticking of the seal faces caused by a poor lubricant between the faces. Will cause vibration problems at the seal face unless the vibration is dampened.
Slurry:
Solids in liquid. It is impossible to define when the quantity and size of the particles becomes too much for the mechanical seal.
Soluble:
When one liquid dissolves or mixes with another liquid.
Specific Gravity:
A measure of the weight of a liquid. Fresh water at 4?C (39?F) is given a value of one. If the liquid you are questioning will float on water the specific gravity is less than one. If it sinks, it is higher than one. Density is a better term.
Specific heat:
Refers to the amount of calories or BTUs required to raise a quantity of a liquid one-degree.
Specific speed:
A formula that describes the shape of a pump impeller. The higher the specific speed the less N.P.S.H. required.
Stainless steel:
Alloy steels containing a high percentage of chromium.
Static head:
The maximum height the liquid is being pumped as long as you take into consideration the siphon affect in some piping systems.
Stationary seal:
The spring loaded or moveable portion of the seal does not rotate with the shaft. Must be used when the seal surface speed exceeds 5000 fpm. or 25 meters per second.
Strainer:
A device installed in the inlet of the pump to prevent foreign particles from damaging the internal parts of the pump.
Stuffing box:
The portion of the pump that held the packing and now holds the mechanical seal.
Stuffing box pressure:
Between suction and discharge pressure but closer to suction pressure.
Submersible pump:
A pump that operates only when totally submersed in the fluid which is being pumped, with waterproof electrical connections, using a motor that is cooled by the liquid.
Suction head:
Exists when the liquid source is above the centerline of the pump.
Suction lift:
Exists when the liquid source is below the centerline of the pump.
Suction recirculation:
Piping from the bottom of the stuffing box back to the suction side of the pump. Used to lower pressure and circulate liquid in the stuffing box.
Suction specific speed:
A formula that will predict one of the types of cavitation. Pumps should be purchased with a number below 8500 (10,000 metric)
Surface speed:
A measure of the feet per minute the seal face or some other component is moving.
Synchronous motor:
Runs with out slippage. Used in elevators and compressors frequently, but seldom used in pumping. Pumps mostly use induction or “squirrel cage” motors.
System curve:
A description of what the pump is required to perform. The pump will pump where the system curve intersects the pump curve.
System head:
The head caused by friction in the piping valves and fittings.