Pumps

Obj 1 types of pumps

• 2 types:

  • Positive Displacement

    • physically displaces out of pump through discharge port.

    • Volume of pump body fixed the amount of fluid displaced.

      • old fashioned well pumps

        • moving piston up and down and has check valves that open or close. Check valve is called a foot valve and it is at the bottom of the pump.

        • Piston must physically move in order for fluid to flow.

      • 2 basic types of positive displacement pumps:

        • reciprocating pumps

          • back and forth, straight line motion.

          • More constant efficiency, combined with capacity flexibility in capacity, head, speed.

          • Can handle variable pressures without speed adjustment or energy consuming throttling devices.

          • 3 types of reciprocating pumps:

            • Plunger Pumps

              • Similar to piston pumps, a plunger moves back and forth in a cylinder by a prime mover and crankshaft (power pump)or by steam or compressed gas. (Direct acting)

              • Has a inlet valve and a discharge valve

              • Distance moved by plunger in the cylinder is the stroke

              • Pumped value is equal to stroke multiplied by cross section area of plunger

              • Piston and piston rid are the same diameters

              • Single acting - one discharge or pumping stroke per revolution.

              • Has pulsations because fluid has to fill the body cavity first then get pushed out

            • Piston Pumps

              • Double acting = area behind the piston can be utilized to do the opposite of what is happening on the front side of the piston. Can be single or double acting depending on how the ports are hooked up

              • Low cost, simple, high pressure

              • Cannot handle abrasives, crystallizing liquids, cannot run dry ir packing is compromised.

              • Operates with positive suction head

              • May be self priming if it has small clearance volumes

            • Diaphragm Pumps

              • similar to piston or plunger but uses diaphragm.

              • Suitable for chemical or hazardous as it seals from rest of pump.

              • Accurate metering of flow

              • Diaphragm may be PTFE Teflon, high pressure or temp it may be flexible stainless or other

              • Hydraulically driven if high pressure allows high discharge press with low dP)

              • Single acting

              • Zero leakage, self priming if good suction lift and clearance. Once primed it can o high suction lifts

              • Can handle air, liquid. Can run dry- convenient if metering liquids out of a barrel.

              • Drawback is smaller capacity (usually 20gpm) and pulsating flow.

              • Often used for chemical feed

        • Rotary pumps

          • rotor turning inside a closed casing with minimum clearance

          • Moves set volume of liquid confined in pumping chambers from inlet to discharge

          • Smoother flow than a reciprocating pump

          • Applications may be:

            • Metering

            • Viscous fluids

            • Hydraulics or slurry

            • Food process

          • Types:

            • gear pump (internal or external)

            • Lobe pump (single, three lobe)

            • Circumferential piston pump

            • Rotary vane

              • rotating cylinder off axis.

              • Spring loaded vanes, motion allows the vanes to vary in each turn.

              • Used often in vacuum installation. (2 stage vane pumps are very common)

  • Variable Displacement (or dynamic pumps)

    • use centrifugal force (force applied to an object being spun around a fixed centre point)

    • Centre of rotation (eye if impeller), pump casing (volute)

    • Rotating the vane to throw fluid outward vis centrifugal force. Creates a area if low pressure and low velocity at eye of impeller

      • velocity energy supplied at the edge of the impeller vane, transforms into pressure energy at pump discharge.

    • Types:

      • Centrifugal or radial flow pump

        • often used in heating/cooling industry (most common)

        • 2 main parts:

          • rotating impeller and shaft

          • Stationary element casing, stuffing box, bearings - known as the collector. Converts liquid to a useable velocity and converts kinetic or velocity energy into pressure energy.

            • 2 forms of collectors:

              • volute: Spiral shaped volute gradually increases in volume. Impeller slows down as pump fills the larger space.

              • Diffuser:

                • stationary diffuser ring is located inside concentric or volute style casing.

          • Suction head: force by weight of liquid

          • Suction lift: partial vacuum created by movement of impeller.

          • radial flow pumps are true centrifugal pumps because there is no axial component to discharge from the impeller.

            • used where a high discharge head is needed.

      • Axial flow pump

        • looks like a axial air fan - it is also known as a propeller pump because of the shape of the blades on the impeller.

          • does not use a volute casing (impractical), uses diffuser vanes instead (may be omitted in extremely low head scenarios)

        • no change in direction of fluid to pump flow.

        • often used in heating/cooling

        • Often used in water treatment plants for storage transfer

        • Often used as deluge pumps in fire suppression.

        • mount vertical or horizontal. Needs to be mounted below the surface of the liquid being pumped.

        • little suction power, needs to be mounted

        • Low discharge pressure but high flow capacity

      • Peripheral flow pump

        • get their name from the pumping action taking place at the periphery of the pump casing

        • concentric casing

        • 2 types: high speed and regenerative turbine.

      • Mixed flow pump

        • in between centrifugal and axial flow. uses centrifugal force and lifting action of vanes on liquid.

        • vertical mount

        • low to medium discharge head

        • high flows

        • volute or diffuser casing.

Obj 2 - Pump Selection Technology

• Force: measure of effort and tends to produce or modify motion. imperial: lb or ton, SI: newtons.

• Density: mass/unit volume. Imperial: lbs/ft3, SI: kg/m3

• Pressure: force per unit area. Imperial: psi, SI: Pa or kPa

• Head Pressure: pressure available at the outlet of a pump or inlet side of a flow conducting system. Expressed in feet of head of water column.

• Vapor Pressure / Boiling Point: Pressure at which a liquid will flash into vapor(aka boil) at a given temperature. Liquid and vapor can coexist in equilibrium at this pressure.

  • vapor pressure increases as temperature rises.

  • Every liquid has a unique vapor pressure vs temperature curve

  • Vapor locked pump: when pumping flow stops due to excessive vapors forming in the pump body.

• Cavitation: pressure of a liquid drops at or below vapor pressure and it forms small bubbles of gas. When the pressure bubbles move to regions of higher pressure they implode. The force of the implosion wears away the material face via fatigue failure over time.

• Aeration: when a pump’s suction inlet has a leak and it sucks up air which is compressible.

  • Reduces liquid that can be sucked into the pump and reduce efficiency.

  • can cause a pump to lose it’s prime

  • if 1% volume is air, it can drastically affect the head-capacity curve. If it hits 6%, it stops working.

• Laminar & Turbulent Flow:

  • turbulent flow (non linear) occurs when particles of the liquid no longer move parallel with pipe axis.

    • caused by bends, abrupt changes in cross section, too high velocity in pipe

    • Halving pipe diameter results in 16x greater frictional losses due to turbulent flow.

• Friction Head (Pressure losses)

  • as liquid flows in a pipe, some hydraulic energy converts into heat and sound by friction of liquid and pipe walls.

    • always present, even with laminar flow

    • If a pipe diameter is halved. the bore area is quartered, resulting velocity is 4x the previous value to maintain the same flow. \

    • Need to calculate the total equivalent length (TEL) to account for all piping losses when sizing pump.

• Suction Head

  • Suction is misleading because pumps do not suck liquid into inlet. Liquid flows in because of a dP between surface of supply reservoir and pump centerline.

  • Positive suction head = weight of liquid + atm press is always available to push fluid into pump and the pump is always primed.

    • Total Suction Head (aka dynamic suction head)

      • Total Suction Head = static suction head (at pump suction) - friction losses from fittings and pipe

• Suction Lift:

  • When inlet supply reservoir is located below the pump.

  • In this case, the pump must create enough suction to lift the fluid up to the pump inlet. This distance from the reservoir level to the pump centerline is the static suction lift.

    • this lift limit is about 20-25 feet (in theory it’s 34ft but due to losses and imperfect vacuum we cannot use that number).

      • most mfg actually limit it to 15 feet.

    • Total Suction lift (aka dynamic suction lift)

      • Total suction lift = static suction lift (at pump suction) + friction losses from fittings and pipe

• Discharge head:

  • static discharge head is vertical distance from center of pump (suction eye) to the surface of the liquid in the discharge tank.

  • Total Discharge Head (aka dynamic discharge head)

    • The head measured at the discharge nozzle. Accounts for the hidden energy expended in moving the liquid from the pump to the end of the discharge line.

    • Total discharge head = static discharge head + velocity head + friction losses from fittings and pipe

      • if discharge piping ends abruptly in a reservoir, the velocity head is lost, if a long taper the velocity can be reduced and the energy can be recovered.

  • Total Static Head

    • distance from liquid supply level to liquid discharge level.

    • Suction Head Version: Total Static Head = static discharge head - static suction head

    • Suction Lift Version: Total Static Head = static discharge head + static suction lift

  • Total Head (total dynamic head)

    • a measure of work being done by the pump in raising the liquid from the level in suction tank to the level in the discharge tank at a given flow.

    • necessary to know to chose a pump. Most often used in industry to describe a pump’s head

    • Suction Head Version: Total Head = Total discharge head - the total suction heat

    • Suction Lift Version: Total Head = Total discharge head + total suction lift

• Net Positive Suction Head

  • amount of energy in the liquid, measured at the centerline of the suction eye of the pump.

  • NPSHR = Required by Pump

    • characteristic of the pump and varies by design

    • energy required at the pump capacity to overcome friction losses between the suction opening and the impeller vanes.

    • determined by lift test from mfg.

  • NPSHA = available in system

    • characteristic of the system and must be calculated to ensure the pump matches the system.

    • energy available in the suction over and above the energy due to the vapor pressure of the liquid.

    • Suction Head, open system → NPSHA = atm press + suction head - friction head - vapor pressure

    • Suction Lift, open system → NPSHA = atm press - suction lift - friction head - vapor pressure

    • Never reduce the suction line pressure to the vapor pressure of the liquid or else you get cavitation. For this reason we use a 10% greater safety factor in calculating NPSHA.

• Piping Systems

  • Open Piping system: pumping where the suction and discharge are both subject to atmospheric pressure

  • Closed Piping system: continuous loop from discharge to suction of pump. Pump becomes a circulator and overcomes friction losses.

    • selection of pump you need to calculate the total pipe friction losses and the required system flow.

    • NPSHR of the pump is to ensure the static system pressure is above the fluid vapor pressure.

    • Important to consider piping components such as expansion tanks. You should have the expansion tank located on the suction side of the pump not the discharge. On the discharge it will reduce the pump’s discharge pressure and possibly cause cavitation due to insufficient NPSHR.

Obj 3 - factors affecting operation of a pump

• Pump Curve

  • division of pump flow vs discharge head. It will run more flow if there is less discharge head pressure to pump against.

  • May show efficiency, brake horsepower, NPSH required for various flow rates.

  • Usually with cold water unless otherwise stated

• Application

  • low to moderate pressure, large volumes = variable displacement pump is needed.

Obj 4 - pump installation requirements

• Inline Pumps

  • allows system water to circulate between rotor and stator - this cools and lubricates bearing minimizing hot spots.

  • motor must be installed in a horizontal position regardless of valve body orientation to ensure bearings are in contact with the system water.

• Frame or base mounted pumps

  • if using rigid couplings, it is crucial for metal frame and concrete base to form stable platform that will not flex under load - otherwise tight alignment clearances will lead to failure of the pump.

• Piping trim required

  • each pump needs these components for proper connection to a piping system:

    • 2 isolation valves

    • strainer - suction side of pump

    • check valve - discharge side of pump - relieves pump of excess head pressure on start or prevent multiple pumps from fighting.

    • 3 pressure taps - 1 upstream strainer, 1 suction of pump, 1 discharge of pump. Helps find differentials of pressure.

  • Big concern for piping is to ensure there is no piping system strain to the pump.

  • Undersized suction lines will also trap air in suction piping and cause damage or erratic behavior. Use eccentric reducers that are flat on top to prevent air bubbles.

• Pump replacement

  • do not drain on floor

  • watch for potential pockets of hazardous gases

  • use pancakes, do not rely on valve for isolation.