Pumps & Systems, May 2008
While complete packaged gas and dry chemical feed systems have been in the water treatment industry for many years, liquid metering pump packaged systems are relatively new. In most cases, plant operators want to purchase a complete chemical feed system-not just a metering pump. Plant or contract engineering staffs can design these packaged systems, and an installing contractor or, in some cases, plant personnel can construct them in place.
In today's environment of shrinking operations budgets and in-house skilled installer staff, the purchase of a single source complete system can greatly reduce capital and operations costs. When procuring such packaged systems, the metering pump design, the fluid pumped and the accessories installed all play vital roles in system safety, repeatability and reliability.
Design Considerations for the Packaged System
The basic chemical feed system contains many smaller components that play important roles in the overall design and operation of a successful, safe and reliable chemical metering package. The application and anticipated system hydraulic conditions determine the material selection, components used, and control and monitoring instrumentation.
Equipment material selection should be based on the metered chemical and the site-specific application. Chemical compatibility determines the material of construction for pipe, fittings, valves and all liquid contact components. Corrosiveness, abrasiveness and solids content of the chemical must first be determined to select the proper system materials of construction.
System component selection should be based on the anticipated hydraulic conditions. Static and dynamic hydraulic forces dictate pump placement and pipe diameters. The hydraulic conditions are derived from the static conditions, elevation of the storage tank, pump location and discharge static head. Dynamic conditions are a function of the chemical viscosity, specific gravity and vapor pressure with respect to the static head hydraulics.
The metering pump location and system components selection are related to the net positive suction head with respect to static and dynamic conditions of pulsating flow. On the suction side, if a pump exerts a negative pressure faster, then atmospheric pressure can force fluid into the chamber, and some of the fluid flashes to gas. Cavitation can damage pump surfaces and reduce capacity and repeatability. For a pump system to function without cavitation, Net Positive Suction Head (NPSH) available must exceed NPSH required.
In addition to NPSH considerations, the metering pump system's pulsating flow will create dynamic head losses due to liquid acceleration/deceleration momentum forces. During each suction stroke in a reciprocating pump system, the entire fluid column between the pump and injection point stops flowing and is forced to start flowing at each discharge stroke. Overcoming inertia can create large momentary pressure spikes on the pump system.
System Component Selection
Like any pump, the metering pump is used to convey fluid and function as a continuous adjustable fluid-measuring device. The mechanism conveying the fluid can be a piston, plunger or diaphragm with an adjusting device to vary the effective displacement per stroke. Motor drive systems can be used to vary the speed of each displacement. With many options available, careful thought must be given to proper selection of the metering and output control devices, all of which can be recommended by the system manufacturer.
Identifying the chemical flow rate and system expected head would be the first part of the selection process. Other factors in pump selection are storage tank elevations, lineal distances of suction and discharge lines, instrumentation control schemes and the overall goal of the feed system.
Calibration columns are used on the suction side of the metering pump to provide actual pump output curves. Manufacturers' data on metering pumps are generally calculated under ideal predetermined hydraulic conditions and are based on metering water. Actual metering pump outputs are determined in the treatment plant using the chemical to be metered under system operating conditions.
Pressure relief valves are a safety device designed to relieve unexpected high-pressure situations due to system obstruction or closed valves.
Pulsation dampeners are used to reduce momentum losses caused by fluid acceleration and deceleration. A pulsation dampener minimizes momentum losses by using compressed air to absorb these forces due to momentum of the fluid inertia. The operation principal is that air is compressible and Newtonian fluids are not.
Backpressure sustaining valves are multipurpose devices providing a constant backpressure on the discharge side of the metering pump. Without constant backpressure, the metering pump output would change with the elevation in the chemical storage tank. The backpressure valve is often used to artificially increase the system backpressure when the suction head is greater than the discharge and chemical siphoning through the system is expected. Also, the backpressure valves work in conjunction with the pulsation dampener, providing a zone of contained constant pressure.