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In development of the piping layout, the design engineer shall determine the size of the suction and discharge piping. For purposes of this discussion, suction and discharge piping refers to the piping immediately upstream and downstream of the pump. The terms suction and discharge header refer to manifolded piping where flow streams from multiple pumps are combined. The suction and discharge pipe sizes for each pump are usually dictated by the flow stream velocity within the pipe when only one pump is operating. The flow where the system head curve intersects with the pump performance curve is usually the worst case flow scenario. The suction and discharge header shall be determined using the pump station firm capacity. Firm capacity is the maximum capacity of the pumps station with the largest pump out of service. The design engineer shall also evaluate the full range of flows including minimum flows and future flows. Based on the process, a minimum flow rate may be required to keep solids suspended in the flow stream. Furthermore, higher flows in the future may be the controlling factor when sizing the suction and discharge pipe sizes.

Figure 2-26: Typical Suction and Discharge Piping 2.4.4.1. Suction Piping

Suction Pipe Size

In order to optimize the suction piping, the design engineer shall select a size which results in a maximum flow velocity between 2 and 5 feet per second (fps) at maximum pump flow rate. Typically suction side velocity is low to minimize the impact on the NPSHa. High velocities induce suction side losses which reduce the overall NPSHa. Furthermore, high velocities on the suction side could result in non-uniform velocity distributions entering the pump impeller, creating impeller imbalance and vibration issues.

Table 2-5: Recommended Suction Side Velocities

Special Applications Recommended Minimum Suction Side Velocity

Solids Handling >2.5 ft/sec

Slurries >5 ft/sec

Mining Slurries Representative sample should be sent to a testing facility to determine the carrying velocity, settlement velocity, shear, specific gravity, corrosivity, and friction factor

Suction Pipe Length

Suction piping must have a straight approach length leading into the pump greater than 5 times the pipe diameter. Straightening vanes may be provided to reduce the approach length, however, the design engineer must consult with the Chief Mechanical Engineer before using straightening vanes in a project.

Figure 2-27: Suction Piping Configuration Excerpt from Hydraulic Institute Intake Design Standard Suction Pipe Fittings

Provide suction piping with eccentric reducers adjacent to the pump. The reducers shall be mounted with the straight section on top, to preclude the trapping of air inside the pipe. (If an eccentric reducer cannot be provided, DO NOT provide an automatic air vacuum release valve because the air valve can leak and admit air into the suction piping – rather, provide a manually operated ball valve for venting air during initial startup.) Eccentric reducers may be mounted directly on the suction flange of the pump.

Suction Valves

The design engineer shall also review the design, for any features which would create an uneven flow distribution in the pump. For example, horizontal split-case double-suction centrifugal pumps include a horizontal suction nozzle baffle. If the pump experiences an uneven flow with the same orientation as the suction baffle, one side of the baffle receives more flow than the other. This uneven flow distribution is detrimental to the pump performance. The source of an uneven flow could be a butterfly valve on the suction piping. As flow passes through a partially open butterfly valve, it is directed towards one side of the downstream pipe creating an unbalanced flow distribution. If the unbalanced flow distribution is aligned with the suction baffle, more flow enters one side of the baffle as opposed to the other side. As a result, the unbalanced flow distribution enters the eye of the impeller. To maintain an even flow distribution into the pump, the suction isolation butterfly valve should be installed with the shaft in the vertical position perpendicular to the pump suction nozzle baffle. The uneven flow distribution is guided downward allowing an equal amount of flow to enter each baffle area.

Figure 2-28: Vertical Turbine Intake Flange Excerpt from Floway Pumps Turbine Pump Handbook Vertical Turbine Pump Barrel/Can Installation

Vertical turbine pumps in a “barrel or can” installation, the suction piping are connected to the barrel.

Flow enters the barrel, and is guided downward entering the suction bell of the bowl assembly.

Where the suction inlet is connected above the suction bell, the barrel inside diameter should be sized for a velocity in the annular space between the ID of the barrel and the OD of the pump bell, bowl or flanged column, (whichever is greater) does not to exceed 3 ft/sec, based on the maximum flow of the pump. For small and medium sized pumps (200 to 2000 gpm) barrel velocities to 5 ft/sec may be applicable. For large applications, refer to pump manufacturer. Mixed flow and propeller pumps should not be barrel mounted due to the high capacities confined to the barrel.

The Hydraulic Institute Guidelines for Intake Design Standards identifies specific dimensional criteria for the suction piping associated with vertical turbine barrel/can installations. This criteria describes the suction pipe approach length and the inlet location relative to the pump suction bell. With regards to the approach length, the horizontal suction piping must have straight approach length (i.e., no flow disturbances such as valves) greater than 5 times the suction pipe diameter. As stated previously, the intent of the approach length is to establish a uniform velocity distribution at the inlet to the barrel. The second dimensional criteria indentified in HIS are the distance between the barrel/can inlet and the pump suction bell. MWH exception to HI, the vertical distance between the centerline of the suction piping and the pump suction bell must be a minimum of 3 to 4 times the pump "can" inside diameter instead of 2D. Refer to the Hydraulic Institute Standards for additional requirements, however HIS requires only twice the can diameter based on MWH experience, additional distance is required in order to allow the flow to straighten before it reaches the suction bell.

The pump “can” or “barrel” must be furnished with air/ vacuum valve in accordance with MWH Standard Detail

M-119.

2.4.4.2. Discharge Piping

The velocity limitations at the suction piping are primarily due to the inlet conditions of the pump. On the discharge side, however, the flow velocities can be higher. The discharge pipe diameter is dictated by balancing the most economical pipe size with the effects of high velocity flow. At a specific flow rate, as the pipe diameter is reduced, the flow velocity increases thereby increasing the head loss through the pipe. It is normally economical to design on-site discharge piping for higher velocities because of the higher average unit costs of the more complex piping and more numerous valves and fittings appurtenant to pumping stations. Velocities leaving pumping units are frequently in the range of 10 ft/sec and it is practical and economical to pass such flows through valves and manifolds before the velocity head is dissipated. The off-site piping is often designed with velocities between 6 and 10 ft/sec. For water conveyance piping longer than one mile, a pump-pipeline economic analysis study shall be performed to determine present worth versus pipe size. A copy of an example pump-pipeline economic analysis study is shown in Figure 2-29.

Figure 2-29: Example Economic Analysis of Pump and Pipeline system

The following table is a guide for velocities to be utilized for water and wastewater pumping plants.

Detailed hydraulic investigations and consultation with equipment manufacturers may result in adjustment to the listed velocities. Reducing the velocities indicated is safe but may result in a less economical design. Increasing the velocities by more than 10 percent, however, should not be undertaken without careful investigation and approval from the Chief Mechanical Engineer.

Total Annual Cost Annual Pumping Cost Annual Pipe Cost Water Velocity