CLEAN WATER – Complete Wet Well-Dry Pit Pump Station Systems
Basic information on the design of this type of pumping system is provided here. Please contact us for more information.
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(Romtec Utilities encourages you to use our Pump Station Design Services. Please contact us to discuss your projects)
INTRODUCTION
For this discussion, clean water is defined as water usually containing very low concentrations of solids and non-aqueous liquids such as oil. Raw water pumped from a body of water is also included in this definition.
Wet well-dry pit pump stations have two distinct vessels: a wet well into which the water flows, and a dry underground vault (dry pit) in which the pumps, valves and other equipment are located. Suction lines near the bottom of the well are connected to the pumps. The well and dry pit are made of precast concrete or fiberglass. These materials can be mixed, i.e. fiberglass wet well or well and a concrete dry pit.
Wet-dry pump stations are occasionally specified for clean water pumping applications. While the pumps used in wet-dry systems are not as efficient as vertical turbine pumps, they are suitable in applications where the pumping rate and total dynamic head are moderate. In addition to the air-cooled pumps usually used in dry pit applications, submersible pumps are frequently specified in wet-dry pump stations, especially in locations where the dry pit could become flooded.
For clean water applications, the principal advantage of wet-dry pump stations over submersible or vertical turbine pump systems is that the entire pumping system can be located underground. Access to the pumps is from within the dry pit. If a pump or other equipment has to be removed for maintenance, it can be lifted out of the dry pit through a hatch located directly above the pumps.
The disadvantages of wet-dry pump stations are that they tend to be more expensive than comparable submersible or vertical turbine pump stations, and they also take up more space.
Intake Structures
Pumps typically do not draw water directly from a body of water. Instead, the pumps are installed in a well located alongside or near the water. Intake pipes connect from the well to one or more intake structures located in the reservoir, lake or river. The intake structure supports and protects the intake pipe.
It may be necessary to divert or screen floating debris from the intake structure and pipe, so it cannot enter the well. This can be accomplished with deflector panels, removable screens or mechanical screening devices, which must be accessible for periodic maintenance.
The water level in the well is the same as the level of the body of water. The well must be deep enough for there to always be adequate water for efficient pumping, even when the body of water is at its lowest level.
A sluice gate is installed in the well at the end of the intake pipe. This makes it possible to drain the well for periodic cleaning, if necessary.
Well
A clean water pump station requires careful design to ensure it will meet the demands placed on it. Romtec Utilities offers numerous designs for the pump station well: concrete or fiberglass, cylindrical or rectangular, vertical or horizontal. The size and depth of the well are largely determined by:
- The size and number of pumps required to deliver water at the preferred rate (gpm)
- The lowest water level in the reservoir, lake or river feeding the well
The exact locations, angles and sizes of the intake pipe(s), suction lines and other items within the well are predetermined and can be prefabricated in a complete package pump station. Coring of concrete or fabrication of fiberglass is done at the factory to eliminate the need for field work and to ensure precise fit of the intake pipes and discharge lines to the well. Flexible seals in all well penetrations are also factory installed.
Other equipment such as a sluice gate and liquid level sensors can be factory-installed or field installed. The well top includes the access hatch with fall protection.
A precast concrete well is delivered to the project site in sections. The base, one or more barrels or shaft sections and the top are fitted with sealing gaskets and are lifted with a crane and stacked in alignment below ground to create the complete well. A fiberglass well can be delivered to the site as a single piece with internal parts fully-assembled.
Romtec Utilities offers concrete and fiberglass cylindrical wells from 4 feet to 12 feet in diameter (ID) and up to 40 feet in depth or length (depending on vertical or horizontal installation). The well in every Romtec Utilities clean water pump station is sized for the specific site requirements, which can be quite varied and include:
- Depths of intake and suction lines
- Current and future peak flows to and demands from the pump station
- Pump sizes (physical size and power rating)
- Relationship between pump area of influence and well diameter
- Consistency of the water level in body of water supplying pump station
Dry Pit
The dry pit is made of precast concrete or reinforced fiberglass. These assemblies are delivered on trucks and installed in the excavated site with a crane.
The entry and exit points for suction lines and discharge lines are pre-cored or fabricated and fitted with flexible seals. Romtec Utilities preassembles and pressure tests the pumps, piping, valves and other equipment, eliminating slow and costly construction work in the field.
The depth, length and width of the dry pit are largely determined by the depth of the wet well, size of the pumps, diameter of the discharge lines and any requirements for special valves, flow metering, bypass pumping port or other equipment located in the dry pit.
Access to all equipment within the dry pit is required, so access hatches, ladders, platforms, lighting and ventilation equipment are also included. Romtec Utilities meets all applicable safety requirements governing confined space entry in the design of the dry pit.
Pumps
Romtec Utilities offers pumps of virtually every type from the leading manufacturers. The specific type and size of pumps in a package wet-dry pump station are determined by many criteria, including: best performance and efficiency, customer preference for a particular brand or type of pump, overall value and other factors.
It should be noted that submersible pumps are frequently used in dry pit applications, especially in sites that may be subject to occasional flooding. These pumps are designed to operate while submerged, so flooding of the dry pit will not damage the pumps.
The pump electrical cables are connected to the pump station control panel through a junction box or pump disconnect panel.
Some wet-dry pump stations have pumps of two different sizes. The smaller pump handles small demand, and the larger pumps operate when the flow into the well or demand for water from the pump station is greater.
In designing a submersible pump station for clean water, the type, size and number of pumps are determined by a variety of factors, including:
- Average and peak demands for water from the pump station or average and peak flows to the pump station
- Total dynamic head (TDH) against which the pumps must work
- Presence of unscreened solids in the water to be pumped
- Possibility of flooding at the site where pumps and well are located
- In some cases, the availability of a detention basin or storage vessels upstream from the pump station
Piping
The types, diameters and lengths of pipes, including the intake pipe(s), pump discharge and force main piping, are predetermined in a package pump station. Romtec Utilities pump stations generally include all piping from the influent through the last valve before the force main. Romtec Utilities designs pump stations with all common pipe materials: stainless steel, ductile and PVC.
It is common to have more than a single discharge point in a clean water pumping system. For example: a pump station that discharges directly to a water treatment plant during the day can be reconfigured to discharge to a reservoir at night. Piping and associated valves for multiple discharge points can be part of the pump station design, depending on various criteria.
Liquid Level Sensors
One or more electronic liquid level sensors are suspended in the well and connected by wire to the pump station control panel. They indicate the fluid level, data which is used by the pump station controller to turn pumps on or off and, in the event of a problem, to turn on alarms and transmit alarm data. For example, if debris blocked the intake line to the point where water could not enter the well, the controller would prevent the pumps from starting. Common types of liquid level sensors are: micro switch floats, multi-electrode probes, pressure transducers and ultrasonic sensors.
The type of sensors used is determined by customer preference or by the controller chosen for the pump station. Many controllers can operate with any type of level sensor, while other controllers are exclusive to one type of sensor. Romtec Utilities designs with all types of liquid level sensors and controllers.
Valves, Pressure Sensor, Bypass Pumping Port
Valves and other equipment in the pump discharge lines are usually located in the dry pit. They can also be located outside of the well in a separate underground vault or above ground if protected from freezing. The package pump station is usually equipped with a check valve and an isolation or plug valve for each pump.
A single non-contact pressure sensor is located in the force main, although some pump systems require individual pressure sensors for each pump discharge line.
Sometimes specified for clean water pump stations is a bypass pumping port for connecting an emergency pump, in the event of a major failure within the pump station.
Flow Metering
Some pump stations must measure the volume of fluid being pumped over a period of time. Flow metering may be necessary for proving the volume. An electromagnetic flow meter inserted in the force main can be located in a separate vault after the valves. To allow maintenance of the flow meter without disabling the pump station, a bypass line with piping and valves to pump around the flow meter is often included in the flow meter vault.
A flow readout unit, usually located at the pump station control panel, receives electronic flow data and reports it to a display panel and a memory device. Transmission of flow data to a remote computer is also available.
Control Panel & Electrical Power
Typically, the electrical controls of the pump station are located in a control panel within a weatherproof enclosure. Various devices provide power to the pumps, control the pumps, receive operational and fault data from sensors within the pumps and the well, report elapsed operational time for each pump and report operational status and alarm conditions both locally and to remote sites through telephone circuits, radio transmissions and other means.
The control panel in a Romtec Utilities pump station can be placed in a variety of locations, generally wherever the customer wants and needs it to be. As an important part of the design process, Romtec Utilities produces a scale drawing showing the equipment layout on the site. No two pump stations have the same equipment layout.
The source and type of electrical power to the pump station site is an important design factor. Romtec Utilities can design the pump station for whatever type of single-phase or three-phase power is available to the site. Generally, the control panel is designed to operate just the pump station. Other electrical demands at the site, such as yard lights and building heat, can be considered in the pump station design when specifically required by the customer.
SCADA
Pump stations can be equipped with SCADA (Supervisory Control and Data Acquisition) devices to both report operational data to remote sites, such as an online computer terminal at the water authority office, and to receive operational instructions and requests for operational data from the same computer or other computers or mobile data devices. This two-way communication capability provides many benefits to pump station operators. Romtec Utilities designs pumping systems to work with all kinds of communications hardware and software.
Electrical Generator
A standby electrical generator that operates the pump station during power outages may be required. The generator is usually sized to operate just the pump station; however a larger generator can be specified for operation of the pump station plus other equipment such as space heaters and exterior lights. Generators using natural gas, propane or diesel fuel can be permanently located at the pump station site and wired to the control panel through an automatic transfer switch that senses the loss and return of grid power and switches the generator on or off accordingly.
The generator can be housed in a sound-attenuating enclosure. The generator can also be installed within a building equipped with a ventilation system. Alternatively, a portable, trailer-mounted generator can be brought to the pump station site and plugged into the control panel to temporarily supply electricity until grid power returns.
Other Optional Equipment
All design parameters to accommodate any additional optional equipment are predetermined in the design of a Romtec Utilities package pump station. The optional equipment is thus integral to the complete pump system design for function, power requirements and other factors. Romtec Utilities specializes in designing pump stations that utilize specialized equipment required by the customer or by various regulatory agencies.
Shelter or Building
A structure to shade the control panel enclosure and provide shelter from weather for operation and maintenance personnel is a good idea, and it may be required. It might also be necessary to house the pump station electrical control panel, generator and other equipment within a building for equipment security and shelter. Romtec Utilities supplies a wide range of shelters and buildings that can be custom fitted to the specific requirements of any pumping system.
