Fluid mechanics problem about selecting pump

ruowan

vd_pumpselection.pptx

Home >Engineering homework help >Mechanical Engineering homework help >Fluid mechanics problem about selecting pump

Southern Methodist University

Bobby B. Lyle School of Engineering

CEE 2342/ME 2342 Fluid Mechanics

Roger O. Dickey, Ph.D., P.E.

V. STEADY PIPE FLOW

D. Pump Selection

Reading Assignment:

Chapter 12 Turbomachines

Section 12.4 – The Centrifugal Pump, pp. 687-700

E. Pump Selection

Pump Applications –

Pumps are used in a wide array of engineering applications including:

Low-lift pumps  high-volume, low-head pumps used to elevate fluids, e.g., elevating water from a supply source to a water treatment plant or wastewater from a gravity sewer to a wastewater treatment plant.

High-service pumps  used to deliver fluids under “adequate” pressure throughout a distribution piping network, or through long transmission pipelines.

Booster pumps  used to increase fluid pressure at intermediate points along transmission pipelines, or within distribution piping networks.

Recirculation and transfer pumps  used to convey fluids for one unit operation or process to another within an engineered system or facility.

Well pumps  used to lift water from ground water aquifers for water supply purposes.

Chemical metering pumps  used to deliver reagent chemicals at precisely controlled rates to chemical processes.

Fire pumps  used for delivering high flow rates at high pressures for firefighting.

Sludge pumps  used to convey thick slurries from one unit operation or process to another within an engineered system or facility.

Sampling pumps  used in both portable and fixed equipment designed to collect precise sample volumes over precise time intervals within engineered systems or facilities.

Pump Types -

Pumps can be broadly classified as either,

Dynamic

Positive displacement

Dynamic pumps deliver flow rates that vary as a function of the discharge head on the pump.

Conversely, positive displacement pumps deliver flow rates that remain relatively constant, regardless of changes in the discharge head.

Dynamic pumps can be further subdivided into classes,

Centrifugal – axial flow, radial flow, mixed flow, and peripheral flow pumps

Special effect – including eductor (or jet), ejector, and air lift pumps

Positive displacement pumps can be further subdivided into classes,

Reciprocating – piston (or plunger) and diaphragm pumps

Rotary – including gear, lobe, screw, progressing cavity, vane, and peristaltic (or tubing) pumps

Centrifugal pumps are the most widely used type in engineering applications including:

Low-lift – Vertical Turbine

Axial Flow

Archimedes Screw

High-service – Split-case, double suction centrifugal

Vertical-turbine Pump

Axial Flow (Vertical Propeller) Pump

Archimedes Screw Pumps

Split-case, Double-suction Centrifugal Pump

Booster

Recirculation and transfer

Well – down-hole pumps

Firefighting

Figure 12.6 – Schematic of Basic Elements of Centrifugal Pumps

Centrifugal Pumps

Submersible

Vertical Sump Pump

Horizontal

Fire Pump System, Internal Combustion Driver

Centrifugal Pumps

Solids Handling, Enclosed Impeller

Non-clog, Open Impeller

Semi-open Impeller

Enclosed Impeller

Open Impeller

Centrifugal Pump Impellers

Down-hole Well Pumps

Special Effect Pumps

Ejector

Positive displacement pumps are commonly used for,

Chemical metering – including diaphragm, gear, lobe, progressing cavity, and peristaltic pumps

Diaphragm Metering Pump –

for feeding chemical solutions

Diaphragm

Positive Displacement Metering Pumps

Rotary Lobe

Progressing Cavity

Gear

Sludge transfer – including piston, diaphragm, and progressing cavity pumps

Sampling – peristaltic pumps

Progressing Cavity Sludge Pump

Rubber Stator

Steel Rotor

Air-operated Diaphragm Sludge Pump

Positive Displacement Sludge and Sampling Pumps

Elastomer Diaphragms

Peristaltic Tubing Sampling Pump

Pump/Piping Systems -

Pump Piping –

Most pumps have isolation valves on both the suction pipe and the discharge pipe. Gate valves, plug valves, and ball valves are commonly used for this purpose. This allows the pump to be isolated from the piping system for maintenance or replacement.

Most pumps have a check valve on the discharge line between the pump outlet connection and the discharge isolation valve to prevent backward flow through the pump when the pump is not operating.

A sketch of a common pump piping arrangement follows,

Gate Valve

Check Valve

Suction Piping

Gate Valve

Eccentric Reducer

Concentric Reducer

Discharge Piping

Pump

The suction pipe for any pump should never be of smaller diameter than the pump inlet connection. If possible, the suction pipe should be 2 or more pipe sizes larger than the pump inlet connection to minimize friction losses.

Recommended, economical velocities for pump suction and discharge piping may be summarized graphically as:

Recommended Velocities

System Head-Discharge Curve

Consider the energy equation around a CS between Sections 1 and 2 that encloses a pump,

Solve this equation for the pump energy input, hP, or “Total Head” required,

Change in Velocity Head ,

Static Head, z

Change in Pressure Head,

Total Head Loss, hL = hf + hm

A plot of the input Total Head, hP , as a function of flow rate, Q, is called the System Head-Discharge Curve for a given pump/piping system.

Consider a typical pump/piping system for transferring water from one tank into a second tank having a higher water surface elevation,

z

Pump

Write the energy equation between the surface of the two tanks,

(i) p1 = p2 = pATM = 0

(ii) V1 = V2 = 0

Simplifying yields,

The total head loss varies with V 2, hence with Q2, such that the System Head-Discharge Curve has the following general appearance,

Total Head, hP [L]

Discharge, Q [L3/T]

z, Static Head

hL , Total Head Loss

System Head-Discharge Curve

Reconsider the head loss equation for a piping system with a single pipe size:

There are usually multiple pipe sizes, having differing fluid velocities, each with different appurtenances in pump/piping systems. The total head loss for a pump/piping system is obtained by summing the head losses for the different pipe sizes:

where,

hL = total head loss for all system pipe sizes [L]

hfi = friction loss for pipe size i [L]

sum of the energy losses for all individual minor loss components j, for pipe size i [L]

Expanding the summation over all pipe sizes:

Minor Losses

Pipe Size 1

Friction Loss

Pipe Size 1

Minor Losses

Pipe Size 2

Friction Loss

Pipe Size 2

Pump Head-Discharge Curve

The discharge delivered by a centrifugal pump typically declines as the Total Head on the pump increases. The Total Head at which the pump discharge is reduced to zero is called the Shut-off Head. A hypothetical centrifugal Pump Head-Discharge Curve, for a single operating speed follows:

Total Head, hP [L]

Discharge, Q [L3/T]

Pump Head-Discharge Curve

Shut-off Head

Other examples of hypothetical centrifugal Pump Head-Discharge Curves are shown in Figures 12.11 p. 693, and 12.12 p. 694 in the textbook.

Example manufacturer’s Pump Head-Discharge Curves taken from the Goulds Pump Catalog are contained in the class handout.

Legend: Goulds Pump Head-Discharge Curves

Efficiency (U-shaped) Curves

Additional information often contained on manufacturer’s Pump Head-Discharge Curves includes,

Pump efficiency

Brake horsepower (i.e., power that must be supplied to the pump input shaft by a drive motor)

Net Positive Suction Head (NPSH) required to prevent cavitation

Head-discharge characteristics as a function of pump operating speed

Head-discharge characteristics as a function of impeller diameter (a given pump casing can often accommodate a range of impeller diameters)

Superimposing Pump Head-Discharge Curves over a System Head-Discharge Curve allows the operating point for a pump/piping system to be established graphically, as the intersection of the curves:

Total Head, hP [L]

Discharge, Q [L3/T]

Pump Operating

Point

QPump

hPump

Pump Head-Discharge Curve

System Head-Discharge Curve

This graphical approach is commonly used to select a suitable centrifugal pump for a given piping system and desired flow rate, Qdesign . A trial-and-error procedure is used. Several different Pump Head-Discharge Curves may be superimposed over the System Head-Discharge Curve until a pump is found with suitable operating characteristics — discharge, head, efficiency, NPSH, etc.

Total Head, hP [L]

Discharge, Q [L3/T]

Operating

Point for

Pump #2

Operating

Point for

Pump #1

Qdesign

Pump #2

Selected

* Important Point

Uncertainty exists in estimating pipe friction and minor losses. Pipe roughness, hence friction losses, may also increase over time due to pipe corrosion or scaling. It is highly recommended that two System Head-Discharge Curves be developed, one for pump capacity selection, and the other for motor selection as follows:

(1) For ensuring adequate pump capacity, Qdesign , estimate the maximum friction loss assuming old, rough pipe using the Hazen-Williams Equation with C = 100. Furthermore, use conservatively high estimates of minor losses by employing minor loss coefficients, i.e., KL values, from the upper-end of typical design ranges for each type of piping appurtenance.

(2) Determine the maximum possible discharge and associated head, (Qmax , hp,Qmax) for the selected pump/piping system. This operating point typically requires the maximum motor power output. Estimate the friction loss assuming new, clean pipe using the Darcy-Weisbach Equation.

Furthermore, use low estimates of minor losses by employing minor loss coefficients, i.e., KL values, from the lower-end of typical design ranges for each type of piping appurtenance.

Plot both System Head-Discharge Curves (i.e., one curve for lowest likely head loss values, and another for highest likely head loss values), and the Pump Head-Discharge Curve for the selected pump on the same graph. Read the design operating point (Qdesign , hp,Qdesign), and the maximum power input operating point (Qmax , hp,Qmax) from the graph as follows:

Total Head, hP [L]

Discharge, Q [L3/T]

Operating Point (Qdesign , hp,Qdesign)

Operating Point (Qmax , hp,Qmax)

Maximum Friction and Minor Losses

Minimum Friction and Minor Losses

Common Pump Piping Arrangements

(1) Horizontal Dry Pit – Flooded Suction

z

Pump

Wet Well

Dry Well

Suction Head

(2) Horizontal Dry Pit – Suction Lift

z

Pump

Wet Well

Suction Lift

(3) Vertical Wet Pit

Wet Well

z

Ground Surface

(4) Submersible

Wet Well

z

Ground

Surface

Valve Vault

(

)



p





All