APPLICATION CONSIDERATION
2A PUMP FUNDAMENTALS
• Types & Classifications of Pumps . . . 2-2
• Centrifugal Pump - Theory of Operation . . . 2-3
• Submersible Definitions & General Information . . . 2-3
• Submersible Pump Performance Characteristics and Curves . . . 2-5
• Submersible Pumping System Overview . . . 2-9
• Submersible - Sizing & Installation Dimensional Relationships . . . 2-13 2B HYDRAULIC FUNDAMENTALS
• Density, Specific Gravity and Weight . . . 2-15
• Pressure and Head . . . 2-15
• Fluid Flow . . . 2-17
• Vapor Pressure . . . 2-17
• NPSH . . . 2-18
• Power, Efficiency and Energy . . . 2-21
• Viscosity . . . 2-22 2C HYDRAULIC RELATIONSHIPS
• Affinity Laws - Pump Speeds . . . 2-24
• Specific Speed . . . 2-25
• Speed Torque Relationships . . . 2-26
• System Head Curve . . . 2-28
• Parallel and Series Operation . . . 2-31
• Minimum Flow - Temperature Rise . . . 2-34
• Axial Thrust - Maximum Flow . . . 2-36 2D PUMPING SYSTEM APPLICATION CONSIDERATIONS
• Cavitation, Vortexing and Submergence . . . 2-38
• Entrained Gas . . . 2-40
• Entrained Solids (Sandy Water) . . . 2-42
• Water Hammer . . . 2-43
• Downhole Check Valves . . . 2-45
• Corrosion . . . 2-46
• Testing . . . 2-52
• Power Consumption and Cost . . . 2-56 2E ENGINEERING PROPERTIES OF WATER:
• Table 2-15: Altitude vs. Barometric Pressure and Boiling Point of Water . . . 2-60
• Table 2-16: Elevations for Various Municipalities (U.S. & Canada) . . . 2-60
• Table 2-17: Vacuum to Suction Lift Conversion . . . 2-61
• Table 2-18: Properties of Water from 32°F to 300°F . . . 2-62
• Figure 2-32: Suction Lift Correction for Elevation . . . 2-63
• Figure 2-33: Suction Lift Correction for Water Temperature . . . 2-63
Section 2
Water Well Pumps. Pumps used for water well service are sometimes classified as shallow or deep well types. A pump installed above the well, which takes in water by suction lift is called a shallow well pump (lift generally
<25’). Typical shallow well pumps are jet pumps and single stage centrifugals with foot valve. A pump installed in the well, with the pump inlet initially submerged below the pumping level is called a deep well pump (positive submergence pump). Typical Deep well pump types are vertical turbines and submersibles.
Deep Well Pumps. Deep wells are generally pumped by multistage diffuser centrifugal pumps commonly called vertical turbines. These pumps develop high head by using a series of small impellers rather than a single large one.
Line shaft vertical turbine pumps may be either oil or water lubricated. Water lubricated pumps are sometimes called open line shaft pumps since the drive shaft is exposed to the flow (Figure 2-2, A). Deep well pumps may be driven either by a motor at the top connected to the pump by a line shaft, or by a submerged motor (submersible) below the pump (Figure 2-2, B).
Figure 2-2: Deep Well Pump Type
Diagram A – Vertical Turbine Pump Diagram B – Submersible Well Pump Figure 2-1: Pump Type Overview
Shallow Well Horiz. Booster Dry Pit Deep Well Vert. Booster Canned Wet Pit Sump Jet
Volute/Centrifugal Split Case
Turbine/Submersible Propeller
Radial Flow Mixed Flow Axial Flow Reciprocating Rotary Dynamic/Centrifugal
(Variable Torque) Positive Displacement (Constant Torque) Pumps
2A PUMP FUNDAMENTALS
Types & Classifications of Pumps
Pumping equipment can be broadly divided into two general categories, positive displacement and dynamic types (centrifugal). Centrifugal pumps are further typed by their general mechanical configuration or by impeller type (ie.
axial flow, mixed or radial flow). The most common centrifugal pump types by mechanical configuration are;
turbine, propeller and volute (centrifugal). The multi-stage submersible is a turbine pump sub-group. The classification relationships are illustrated below in Figure 2-1.
Section 2 Centrifugal Type - Submersible Pumps. In a centrifugal pump, pumping action is generated by means of
centrifugal force. A submersible pump is a multi-staged centrifugal pump. The essential components of a
submersible pump are the intake, impellers, pump/bowl shaft, diffuser and discharge; all driven by a electric motor prime mover. Each rotating impeller and stationary diffuser element is commonly referred to as a stage or
intermediate. A simplified submersible diagram is illustrated in Figure 2-3 below:
Centrifugal Pump - moves faster at the tips of the impeller than at the eye. The presented in this manual is specifically applicable to the enclosed - fixed type impeller configuration.
Submersible Pump Definitions and General Information
A submersible pump can be broadly described as a pumping unit in which the pump and its driving motor operate submerged in the liquid being pumped. This broad definition applies to many pump types; however, the focus of this manual is on the deep well submersible pump with enclosed - fixed type impellers.
A deep well submersible pump consists of a multistage centrifugal pump directly coupled to an electric-motor which is designed to operate completely submerged in cool water. Generally the pressure or pumping depth capabilities vary with the number of impellers used and the capacity varies with the impeller design. Most
submersibles utilize a 2 pole - 3600 rpm (ideal speed) motor designed to fit typical water well diameters (4” - 20”).
Domestic sizes range from 1/4 to 2 Hp and commercial sizes are available to 250 Hp and larger, for special applications. The advantages are low cost per gallon of water pumped, ability to be installed at great depths, quiet operation, ease of installation and low over all installation cost especially for deeper settings. Priming is automatic
Figure 2-3: Submersible Pump Elementary Overview Diagram
Motor Shaft
Section 2
and maintenance is no problem. The primary disadvantage of a submersible pumping unit, assuming good design practices are followed, is the necessity to pull the entire pump regardless of problem (electrical or mechanical).
Figure 2-4 illustrates a typical form of construction and associated terminology used in the manufacture of
submersible pump-motor units. This assembly shown is of the threaded bowl/intermediate construction, employed by Grundfos, although bolted construction is used by many pump manufacturers.
Figure 2-4: Typical Multistage Submersible Pump Construction Sectional Drawing
17
10 Bottom intermediate chamber with stop ring 11 Nut for split cone
Pump connection to motors of different sizes is typically accomplished through the bolting of the inlet adapter (suction interconnector) to the motor bracket. Shafts are typically stainless steel or other corrosion-resistant material.
Intermediate stages (bowls), impellers and other fabricated/cast components may be of cast iron, bronze, molded thermoplastic, stainless steel or special alloy, depending on the operating environment and pump application.
Impellers may be mounted to the bowl/pump shaft through the use of a key, formed shaft, (spline shaft), split cone or lock collet. Rubber and/or bronze bearings are typically used in water supply applications. Most manufacturers offer a variety of designs, features and material options. Specific product information is normally provided by the manufacture in catalog format.
Section 2 Submersible Pump Performance Characteristics and Curves
General. Centrifugal pumps have head-flow characteristics, just as motors have speed-torque characteristics. At a fixed speed, the head developed by a pump will decrease as the flow is increased. Different pump designs will produce different characteristics, as illustrated in Figure 2-5 below:
Reading a pump curve is manufacturers. Typical performance curve presentations are illustrated in Figure 2-7 and describes the relationships between (1) capacity and total dynamic head, (2) capacity and efficiency, (3) capacity and brake horsepower, and in some cases, (4) capacity and net positive suction head (NPSH). Individual curve parameters are discussed below.
Performance Curves
1. Total dynamic head- capacity (H-Q) curves show the total head developed by the pump at a given capacity.
Figure 2-6 shows that a pump will operate over conditions ranging from shutoff (no flow) to maximum flow.
Maximum total head usually occurs at shutoff. As capacity increases, total head developed decreases. Maximum flow will occur with minimum head.
The characteristic curves for a multi-stage turbine type submersible pump depend upon the number of stages or impellers. Each impeller normally will have the same characteristic curve as the next, the composite curve is obtained by adding the head per stage at a given discharge (flow rate) to determine the effect of series installation.
The H-Q performance curves for engineered submersible pumps are typically show the head developed by one stage. If a multistage pump is used, the total head developed at a particular capacity, (based on a single stage performance curve) can be calculated using the formula:
where; H = total head of pump, n = number of stages, Hs = head per stage.
2. Efficiency-capacity (PE - Q) curves describe the relationship between pump efficiency and capacity. Efficiency is maximized at the design capacity where hydraulic, mechanical, and leakage losses within a pump are minimum.
These losses included leakage between impeller and intermediates; fluid friction losses in all flow passages such as diffuser vanes, impeller, intermediates and thrust bearing friction. If the pump operates at capacities greater or less than at the design capacity, pump efficiency will decrease. The efficiency may change as more stages are added. Efficiency corrections for multistage pumps are provided with the manufacturers performance curve.
3. Brake horsepower-capacity (BHp-Q) curves show the brake horsepower required by the pump at a given capacity within its performance range. They can be used to select and properly size a motor, as well as quantify the impeller loading characteristic as nonoverloading or overloading. In the nonoverloading case, BHp varies Figure 2-5: Pump Characteristic Illustration
slightly over the pump’s with overloading performance curves are rarely used in submersible water supply applications.
The type of BHp-Q curve depends primarily on the impeller design; (1) Radial flow impellers usually have overloading curves where BHp increases as capacity increases (2) Axial-flow impellers also have overloading curves; however, BHp increases as capacity decreases and maximum BHp occurs at shut off (3) Mixed-low impellers generally have non-overloading curves.
The total BHp requirement for a multistage pump, (based on a single stage performance curve) can be calculated using the formula:
where; BHp = required BHp for multistage pump, n = number of stages, BHp/stg. = Hp required by one stages.
4. Net Positive Suction Head - Capacity (NPSH-Q) curves show the required NPSH (NPSHR) for a particular pump design to operate without cavitation. Pump NPSH requirements increases as capacity increases. Pump NPSH requirements are determined by the manufacturer. The topic of NPSH is discussed in detail in Section 2B.
In addition to the minimum parameters described above, submergence minimums and hydraulic thrust data are often presented on the performance curve for engineered products. Many manufacturers feature a single stage curve with head and capacity (H-Q) shown for full-sized impeller and for one or more trimmed (reduced diameter) impellers. Other manufacturers present a cluster of curves where capacity and head are shown for each stage up to the maximum permissible number of stages. The closed impeller design (upper and lower shroud) is most commonly used in submersible applications. Semi-open and axial flow impellers can not be effectively used in submersible applications as axial (lateral) clearance adjustments for optimum performance are difficult to maintain.
Section 2
Figure 2-6: Elementary H-Q of Performance - Multistage Turbine Type
FLOW
Section 2
Figure 2-7: Typical 8" Submersible - 3600 rpm Multi-Stage Performance Curve
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
0 50 100 150 200 250 300 350 400 450 500
CAPACITY (GPM)
HEAD (FEET)
80
70
60
50
EFFICIENCY (%)
385S1000-13 (100 HP)
385S1000-12 (100 HP)
385S1000-11 (100 HP)
385S750-10 (75 HP)
385S750-9 (75 HP)
385S750-8 (75 HP)
385S600-7 (60 HP) *
385S500-6 (60 HP) *
385S400-5 (40 HP) *
385S400-4 (40 HP) *
385S250-3 (25 HP)
385S200-2 (20 HP)
385S100-1 (10 HP) *
EFF. (%)
3450 3525
OPERATING RANGE: 260 TO 550 GPM RPM
CAPACITIES BELOW 300 GPM SEE MODEL 300S
Section 2
Note: When a pump is connected to a system, the system pressure will dictate the natural operating point for the pump.
Performance - Speed Relationships. Many manufactures who build both submersible and turbine pumps will provide H-Q performance based on the speed of the surface motor driver. Submersible motors have slightly lower full load speeds compared to comparable surface motors as a result of their compact design. Typical 2-pole speeds for surface and submersible motors range from 3570 - 3500 rpm and 3540 - 3450 rpm respectively. The resulting H-Q differences can be neglected in most applications. Should a more precise H-Q representation be required, the performance curve can be derated using the affinity laws.
Note: Speed difference also exists between submersible motor designs and sizes. Always verify the pump performance curve basis of calculation with the actual full load speed of the motor to be used.
Shape of Pump Curve. There are three types of H-Q curves steep, flat and drooping. Steep curves are characterized by a large change in total head between shut off and capacity at maximum efficiency, while a small change occurs for flat curves. Drooping curves are characterized by an increase in total head to some maximum value as capacity
increases, then a
Figure 2-8: Typical 8" Submersible - 3600 rpm Single-Stage Performance Curve
0 50 100 150 200 250 300 350 400 450 500 550
385 GPM, Single Stage Curves, 60 Hz
possible discharge rates at the same head can cause a system to “hunt” back and forth between capacities.
Performance curves also may have irregularities or flat regions which can cause unstable performance if the pump operates within the unstable region.
Radial-flow (low specific speed) impellers generally have flat H-Q characteristic curves, while axial-flow (high specific speed) impellers have steep curves. Mixed-flow impellers have intermediate characteristics. In most water supply applications pumps with moderately steep characteristics are preferable.
Hydraulic Characteristic and Curve Standards. The head (in feet of liquid) developed by a centrifugal pump is independent of the specific gravity, water at normal temperatures (60 - 70°F) with a specific gravity of 1.00 is the liquid almost universally used in establishing centrifugal pump performance characteristics. If the head for a specific application is determined in feet, then the desired head and capacity can be read without correction as long as the viscosity of the liquid is similar to that of water. The horsepower (BHp) curve, which is also based on a specific gravity of 1.0, can be used for fluids other than water (if viscosity is similar to water) by multiplying the horsepower for water by the specific gravity of the liquid being handled.
The hydraulic characteristics of centrifugal pumps usually permit considerable latitude in the range of operating conditions. Ideally, the design point and operation point should be maintained close to the best efficiency point (BEP); however, substantial variations in flow either to the right (increasing) or to the left (decreasing) of the BEP are usually permissible, operating back on the curve at reduced flow, or at excessive run out may result in radial thrust, or cavitation causing damage.
For pumps in the centrifugal range of specific speeds (radial flow impellers) the relationships between capacity, head and horsepower with changes in impeller diameter and speed can be predicted using the affinity laws. The affinity law topic is discussed in detail in Section 2C.
Submersible Pumping System Overview
Application. The submersible pump is especially suited to deepwell and booster service for industrial, commercial, agricultural and municipal water systems. The pump utilizes a submersible induction motor coupled directly to the pump end (bowl assembly) and is designed to operate completely submerged in the fluid being pumped. Power is supplied to the motor through waterproof electrical cable. In deepwell applications the pump, motor and cable are suspended in the well by the column (riser) pipe. Booster applications involve installing the unit vertically in a barrel (can) or sump, or horizontally in a pipe line or tank. Since the entire unit is either enclosed or below the surface of the ground, there are several applications where the submersible pump has many advantages when compared to line shaft vertical turbines. These advantages and application are:
• Deep settings and high head • Crooked wells
• Quiet and vandal resistant • Low initial equipment and installation costs
• Vertical or horizontal application • Unaffected by weather extremes
• No routine maintenance required • Minimum space requirement
• Small diameter - high flows
High temperature (100°F +) and abrasive environments are generally not conducive to submersible applications. In such cases, the line shaft vertical turbines are usually more suitable.
Typical Operation. Submersible pumps may be operated and controlled in the same manner as any other types of turbine pumps in similar applications. No special consideration peculiar to the submersible is generally necessary, with the exception of the motor starting equipment. The motor, being installed in the pumped fluid, may not be subjected to the same ambient temperature as the overload relays in the starter.
Submersible Pumping System Component Overview. Submersible pumping system equipment requirements can be broken down into two component categories, sub-surface and surface. A typical submersible pumping system is shown in Figure 2-10.
Section 2
Section 2
The major sub-surface components are:
• Submersible Motor • Pump End (Bowl Assembly)
• Power Cable • Check Valve
• Column (Riser/Drop) Pipe The major surface components are:
• Starter Panel (Controller) • Surface Discharge
• Surface Discharge Piping • Transformers
A brief description of the major functional components, as well as general selection and application criteria guide lines are presented as follows:
Submersible Motor. The electric motors most commonly used in submersible pump water supply applications are two pole (3600 rpm), 3 - phase, squirrel cage, induction type - operated at 60 Hz. Motor construction is typically of the hermetically sealed - canned type, in which the winding is insolated from the motor liquid (de-ionized water).
Motor liquid is required to dissipate heat and for thrust bearing lubrication. The motor is attached to the pump end assembly via a coupling and bolted interconnector, which creates integrated submersible unit. The motor thrust bearing carries the entire thrust load of the pump. See Section 4A for a detailed discussion of submersible motors.
Submersible motors rely on fluid movement over the external housing to remove heat. The primary factors which contribute to early motor failure are; insufficient or lack of cooling flow, prolong low voltage operation and high ambient fluid temperature. Mitigation of adverse motor operating conditions are discussed in Section 4B under the general heading of motor cooling.
Pump End. The pump end (bowl assembly) consists of single or multiple stages to meet exact system requirements.
A wide range of pump end sizes are available to meet system capacity requirements. Grundfos standard
construction utilizes all stainless steel construction and industrial grade rubber (NBR) seals and bearings. Stainless steel standard construction allows for a greater range of application when compared to cast iron - bronze fit (CIBF) standard construction. See Section 5 for a detailed presentation of Grundfos submersible pumping products.
Submersible pumps are relatively trouble free under most operating condition. When problems do arise, they can usually be attributed to inadequate suction/intake condition or the presence of sand (abrasives) in the water.
Mitigation of adverse pumping conditions are discussed in greater through out this section.
Power Cable. Power cable is used to transmit power from the starter (controller) to the motor and is selected according to load, voltage and length required. One extra foot for each fifty feet of length should be allowed, plus an additional ten to fifty feet for surface connections. Electrical losses in the cable contribute to reduced overall plant efficiency, and for this reason it may be advantageous to oversize cable on some installation (year round operation - deep setting). Cable is typically supported on column pipe by means of cable clamps and stainless steel bands, nylon ties or tape.
Check Valves. Column check valves are recommended for pump settings in excess of 450 feet. For pump settings in excess of 750 feet two check valves are recommended. The bottom check valve should be located 40 to 60 feet above the pump, if the pump is not equipped with a built-in check valve.
In no case should the distance between check valves and the surface discharge plate be equal. Unequal distances are essential to prevent harmonic valve hammer. It is recommended that column check valve(s) be utilized on any installation where there is danger of pump start during back spin or danger of well damage as a result of surging created by rapid column drainage. In cases where it is desirable to drain the column pipe - a slow leak check valve should be used in conjunction with a backspin timer.
Care must be used when installing a check valve on the surface or within the well above the static water level. A implosive vacuum can form if water recedes rapidly down the column pipe or destructive water hammer can occur as the void created by the vacuum is filled. A vacuum/air relief valve should be considered for installation in the discharge header or a snifter (air inlet) valve located in the upper most column joint as a fail safe should a down hole check valve fail. The air relief/inlet valve should be placed on the downstream side of a surface check valve or
Section 2 Brass plug drain valves are available for small diameter column pipe. Drain valves are sometimes use to prevent the
Section 2 Brass plug drain valves are available for small diameter column pipe. Drain valves are sometimes use to prevent the