Engineering of Water Systems

Part 15(d)—Well Pumps as Booster Pumps, Closed Systems

By Ed Butts, PE

The past three columns of The Water Works series have discussed using vertical turbine and submersible pumps in parallel and series pumping installations. In the July column, we provided an overview on using these types of pumps in an open system—in other words, the water source is exposed to normal or atmospheric pressure only. This month, we’ll introduce the concept of using vertical turbine or submersible pumps in a closed system, generally using a pressurized suction environment with pump cans or barrels.

Pressurized Booster Pumps (Inlet Pressure)

As outlined in an earlier column, a booster pump can consist of a conventional end-suction or split-case centrifugal pump as well as a vertical turbine or submersible well pump. The selection choices are almost endless.

The primary criteria when applying a centrifugal pump to a booster application is to design the pump to the same basic hydraulic condition (flow rate) as the source pump or supply and verify the combined pressure of both pumps or systems will not exceed the maximum working pressure of the booster pump or pump can.

The other consideration for using a centrifugal pump as an inline booster pump is to design the inlet and outlet piping arrangements to provide as orderly and efficient a piping as possible.

Although a few varying conditions do apply, for the most part the same application and design guidelines applying to a conventional well pump will also apply to using the same pump for pressure boosting.

The primary differences with a vertical turbine pump (VTP) involve positive inlet and outlet pressures, discharge head configuration and construction, and shaft sealing methods. As opposed to a submersible pump, a VTP can be configured to permit attachment of the suction source directly to the inlet (suction) port of the bowl assembly. I have personally used this method several times with a VTP with the discharge head and motor situated on an upper floor, free from potential flooding, and the lower bowl assembly attached directly to a piping manifold from a reservoir or pressurized source.

In other cases, depending on the style of discharge head and pump can, the designer must consider the annular velocity of water traveling past the bowl to enter the pump. If this annulus is insufficient, excessively long, or obstructed, the resulting increase in velocity and turbulence can impact the suction conditions to the pump.

A linear and smooth passage between the pump can and bowl assembly, one with a velocity of 5 feet per second (FPS) or less, is preferable.

Another factor involves sealing of the pump’s base to the pump can. When used in pressurized suction sources, various methods exist to perform the sealing between the pump can and discharge head. Although many customized and manufacturer-standard assemblies exist for this purpose, I have found using a matched standard flange size, configuration, and drilling on the head and can works best. Since most pump manufacturers offer this type of construction, either in a standard or optional configuration, and flange pressure ratings range from ANSI 125- to 250-pound design and drilling, obtaining a matched unit is not generally difficult.

Sealing of the pump shaft through the stuffing box, especially when using conventional packing, can be a significant issue. Since much of the sealing effect with packing is facilitated during operation as the spinning of the shaft often creates a condition where shaft sealing is fairly predictable, once the unit has shut down the remaining static head from the source tends to allow seepage, wicking, or even squirting of fluid
through and between the stuffing box and shaft. This often results in the loss and buildup of a considerable volume of fluid onto the discharge head and adjacent floor surfaces.

Normally, this condition when using packing is difficult—if not impossible—to control and the designer should either plan for the continuous drainage and discharge of this extraneous fluid, or as an alternative, use a mechanical seal. In most cases when a VTP is used as a booster pump, I recommend either a single or double mechanical seal be employed for shaft sealing on potable water booster pumps.

In normal situations and pressures, a single mechanical seal will work well for this application. Using a two-piece headshaft and coupling between the discharge head and driver facilitates removal and service of the seal without needing to remove the driver.

In other cases with abrasives, use of a double mechanical seal works well for the same purpose, but with a double seal a source of lubricating fluid at a pressure higher than the booster pump operating pressure must be routed through the seal cage to maintain cooling and lubrication of the seal. This often requires the addition of a small booster pump.

Pump Cans

A viable alternative to using vertical pumps in an open, atmospheric head arrangement is through use of a pressurized suction system, typically using a canned pump. The use of vertical multistage pumps, vertical turbine pumps, or submersible pumps in a pump can is a viable and common method of boosting water pressure (elevating water to a higher level).

Figure 1. Pump can hydraulics.

As opposed to most open sump arrangements pumping water from a reservoir or open body of water, a pressurized booster pump can’s inlet needs to be only as deep as the pipe serving it. This method, in addition to often using off-the-shelf components and consuming less floor space than many other conventional booster pumps, is also an efficient method—especially at heads exceeding 150 feet with bowl efficiencies often exceeding 80%.

When used in water well applications, use of the circular well casing controls the diameter of the bowl and column assemblies as well as velocity past the bowl when top-feeding. Similarly, when used for a pump can, the can’s diameter is generally controlled by the maximum bowl diameter, plus when top-fed, the annulus size between the bowl and pump can necessary to control the fluid velocity to less than 3-5 FPS.

A pump can is a fabricated assembly, usually cylindrical in nature, that can be configured in many ways to accommodate the suction piping.

Design Factors: Hydraulic and NPSH

As opposed to vertical pumps in an open sump arrangement influenced by and exposed only to atmospheric pressure and gravity flow associated with the altitude and head differential, a pressurized pump can relies on the supply pressure exerted from the water source (or supply pump) into the pump can.

Figure 2a. Type I pump can.

Although this suction head for pressure boosting is often 20 psi or greater, pump cans can also be applied in instances using open source reservoirs where the inlet head is generated strictly from the vertical head within the reservoir (see Figure 1).

The pump shown in this figure obtains its inlet or suction head from the combined pressure exerted from atmospheric pressure plus the developed water head into the first stage (lowest) impeller. Regardless of whether the pump can is fed from an open reservoir with 100 feet of vertical column head or an independent pressurized water supply of 43 psi, the available pressure or inlet head into the first stage is the same and the paramount concern.

As with any pump under a flooded suction, the NPSH available (NPSHA) is developed from the sum of atmospheric pressure (ha) plus the standing head of water (hs) less vapor pressure (hvpa) and frictional loss (hf) into the can and pump.

In these cases, the static inlet head may be as low as 5-10 feet. Therefore, proper consideration of the combined conveyance and pump can frictional loss become much more critical. Pressurized pump cans are usually provided with much more inlet head to the extent that the NPSH is not usually a factor. Pressurized pump cans are often used for series pressure boosting where the pressure developed by the booster pump at a given flow rate is added to the residual supply pressure at the same flow from the source to generate a higher delivery pressure or head.

The primary design element in this column involves maintaining the proper net suction or inlet head to the booster pump. The head losses external to a pump can are usually deducted before this value, using standard head loss values for the appurtenant type and size of pipe and fittings.

In lieu of specific information, the following equations can be used to approximate the head loss of water once it has entered a top-fed, tee-type pump can; is passing by the bowl; and approaching the pump inlet. Obviously, the K factors are approximate, but they are deemed as reliable and accurate for most uses:

Equation for 90° inlet head loss:

hf= (K)   K = 0.50

Equation for water passing and turning into the bowl:

hf = (K)         K = 3.50; V = approach velocity in FPS

Figure 2b. Type II pump can.

64.4 V = approach velocity in FPS

Example (referring to Figure 1):

Given that the reservoir at sea level has 16 feet of 60°F water over the VTP pump’s first stage (hs), the pipe size is 6-inch steel between the reservoir and pump with 6.5 feet of pipe distance and flow equals 525 GPM, determine:

(1) the total head losses and inlet head to the bowl

(2) the NPSHA

Solution:

(a) Determine velocity: flow = 525 GPM/448.8 = 1.17 CFS/0.196 ft2 (6-inch area in ft2) = 5.97 FPS

(b) Vapor pressure for 60°F water = 0.59 feet (from water characteristics table)

(c) Determine head loss from reservoir to bowl and the net inlet head into the first stage:

Projection into reservoir:

(Entrance loss) = 

Velocity head:

(d) Use equivalent length method for pipe and fittings:
(2) 6-inch, 45° ells = 7.1 feet each × 2 =14.2 feet + 6.5 feet = 20.7 feet of EL

Using Hazen-Williams: 20.7 feet EL at 3.8 feet/c (C=100) × 0.207 = 0.787 feet

(e) Determine can head loss: 90° entry into discharge head:

(f) Total hf = 0.44 feet + 0.55 feet + 0.787 feet + 0.275 feet + 1.94 feet = 3.99 feet

Inlet head to bowl = 16 feet – 3.99 feet (hf) = 12.01 feet

(g) Determine NPSHA:

NPSHA = atmospheric pressure (feet) – vapor pressure (feet) +/– water head – head losses

NPSHA = 14.7 psia × 2.31 = 33.96 feet – 0.59 feet + 16 feet – 3.99 feet

NPSHA = 33.96 feet – 0.59 feet + 16 feet – 3.99 feet = 45.38 feet

Design Factors: Structural

The fundamental design element for a pressurized pump can involves the wall thickness needed to withstand the maximum internal pressure. Typically, either steel or ductile iron is used for pressurized pump cans. However, alternate materials such as PVC or ductile iron have also been used for lower pressures. The basic determination of the required wall thickness is a factor involving the outside diameter and allowable unit stress and is ascertained through an equation called Barlow’s Formula shown below:

Required wall thickness (t) (in inches) =

Variations on this formula are: or or

where:
P = Internal maximum design or operating pressure, psi (for head applications: feet of head × 0.433)

Figure 2c. Type III pump can.

D = Outside diameter of can in inches

S = Allowable unit stress, psi (for A53-grade B steel: 17,500 psi; for A536 ductile iron: 21,000 psi)

t = Required wall thickness in inches

It is important to note these values reflect the maximum allowable unit stress in the applicable can material only and do not include factoring or derating for corrosion or the method of connection to flanges, fittings, joints, etc. Where pressurized cans are to be used, an additional derate for weld or joint efficiency should be applied.

In addition to the required minimum wall thickness determined from Barlow’s Formula, prudent designers will usually add an appropriate wall thickness as a safety factor to allow for potential corrosion or common variations in commercial steel pipe thicknesses or material inconsistency. In my case, for metallic materials I routinely add a minimum of 1/16 of an inch (0.0625 inch) to the wall thickness result.

Figure 2d. Type IV pump can.

Although the use of Barlow’s Formula provides a reliable and reasonably accurate determination of wall thickness for circular pressurized vessels or pipe, many applications require the further consideration of environmental or exposure factors as additional design factors. These factors are usually used where American Society of Mechanical Engineers (ASME) codes apply and are described as Class 1 through Class 4 locations with applicable design factors ranging from 0.72 to 0.40, respectively.

Inlet and Outlet Configurations

Inlet and outlet configurations for pump cans are often variable in design and depend heavily on the piping arrangement and type of pump. Vertical turbine pumps may use a tee head where the pump suction (inlet) and discharge (outlet) are opposed to each other on a common discharge head at a 180° orientation or a can inlet where water enters the pressurized can and the discharge head consists of a flanged base. In addition to canned configurations, a threaded suction inlet on a vertical turbine pump can also be connected directly to the source water.

I have applied this method several times, notably when the bowl was placed into an open gallery where the pump’s inlet could be connected directly to the suction supply, and even in cases where the pump is exposed to a suction lift. In these instances, verification of the appropriate NPSH factors is mandatory.

Various methods exist for constructing pump cans and each one has specific velocity and flow characteristics that must be observed. Refer to Figures 2a–2d for examples of various pump can configurations.

Placing multiple pumping units on a common suction manifold is generally not a risky proposition, so long as the proper precautions are observed. See Figure 3 for an example of the correct sizing and pump location.

As opposed to many other types of underground piping, pump cans are generally installed for the long haul, assuming they will not be retrieved or replaced over the life of the structure. This is due to the fact many are placed inside of permanent structures where access to repair a leak or replace a pump can would be problematic and expensive.

For this reason, pump cans, particularly those fabricated from welded steel, should be completely sandblasted to bare metal and thereafter coated on the interior and exterior surfaces following fabrication. There are many excellent coatings available for this purpose. In my opinion, though, the coating should consist of a fusion bonded epoxy (FBE) that will adhere to the metal surfaces and not experience future chalking or degradation.

Special Considerations

Using a vertical turbine or submersible pump unit as a booster pump creates a few other considerations. When used in a pump can there are generally two primary considerations:

Figure 3. Multiple pumps.

(1) Using a can with sufficient depth and diameter to provide adequate pump (bowl) and motor clearance as well as enough annular clearance to provide low head loss water passage past the unit when top fed.

(2) Providing adequate sealing mechanisms on the top of the can to seal the motor cable (for a submersible unit) or provide a drainage route for water leakage through the stuffing box/packing to relieve the static pressure during unit shutdown when a VTP is used. A single mechanical seal does not generally require a drainage path since zero leakage is more common. A double mechanical seal requires a source of greater pressurized potable water than the pump pressure.

Use of a submersible pump in a pump can also requires proper and effective sealing of the cable as it exits the pump can. The sealing of the motor power cable can be problematic, especially in cases with a moderate to high inlet pressure, which often requires use of an epoxy poured connection at the top of the can. The use of a round jacketed cord (Type SJO or W), rather than individual conductor strands, generally provides a much better seal.

If a junction box is placed on the top of the pump can to facilitate splicing of the motor conductors to the offset conductors, the junction box should be 12 inches in minimum height and provided with an adequate drainage path from the bottom of the box to allow relief of any extraneous water and leakage that may occur through the seal before water can rise in the box enough to contact and invade the splice. For a submersible pump/motor unit, two additional considerations are needed—maintaining adequate cooling flow past the motor ensuring proper grounding of the motor.

Figure 4. Submersible pump can with motor shroud, grounding, and proper cable sealing.

For a top-fed pump can, providing adequate cooling velocity past the motor generally entails using a shroud that encompasses the motor and possibly the pump as well. This forces water to flow to the bottom region of the can and flow upward to the pump inlet, ensuring a cooling flow is generated past the motor.

The velocity of this flow is also critical and dependent on the motor’s diameter and horsepower with a requirement of 0.25 feet per second past 4-inch motors, 0.50 FPS for most 6-inch and 8-inch motors, and up to 0.80 FPS for 10-inch and larger motors.

Balancing this required motor cooling velocity along with maintaining an annulus velocity between the pump can and shroud to a value as low as possible to avoid excessive head loss and turbulence is a design factor that must be carefully observed. See Figure 4 for an example of a submersible pump can with a motor shroud and grounding and proper cable sealing at the top.

_____________________________

This concludes this edition of The Water Works. In the next installment, we’ll continue this series on the basics of engineering water systems with a two-part overview on pump driver design.

Until then, keep them pumping!

Learn How to Engineer Success for Your Business
 Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE. Click here for more information.

Ed Butts, PE, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.

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