Pumps & Systems, March 2008
Many items must be considered when designing pump station control systems with power requirements, level control method and control panel location often among the most important.
Power Requirements
The pump horsepower and power available at the site must be known to prepare a comprehensive control package. The pump horsepower-in combination with the line power amplitude-determines the motor-starting means and branch circuit protection requirements. For medium-to-small horsepower applications, "across the line" starters are used most often because of their simplicity, low cost and reliability.
Starters and contactors are available in several different styles: NEMA (National Electrical Manufacturers Association), IEC (International Electrical Commission) and definite purpose type. NEMA starters typically provide longer performance than their IEC and definite purpose counterparts, but can be initially more expensive. There are also several versions of manual starters which may be used with a contactor to provide automatic motor control.
Starters differ from contactors in that starters have some type of overload relay between the contactor and the load for overcurrent protection. For fractional horsepower single-motor applications, an overload relay may not be required because a thermal switch, which opens when the motor is overloaded, will likely be installed in the motor assembly for protection.
For larger pump motors, some type of reduced voltage starter may be necessary. Many state and local codes require reduced voltage starters for ratings above 20-hp at 208/230VAC or 40-hp at 460VAC to reduce line power voltage sags when a large inductive load is connected. Reduced voltage starters are either electromechanical or solid state.
Electromechanical reduced voltage starters are available in several different versions. The most common are auto-transformer, wye-delta (star-delta), part winding and primary resistor. These starters are reliable and some are designed to accommodate a specially-wound motor. However, they tend to be large and expensive, especially when compared to a solid state reduced voltage starter.
A solid state reduced voltage starter (SSRVS) is just that-a starter. An "up to speed" bypass contactor is typically used in conjunction with the SSRVS to divert the motor current so that the solid state components do not have to handle the full load for extended run times, therefore prolonging the device life. Many manufacturers integrate the bypass contactor on small horsepower solid state starters.
A SSRVS has advantages over the electromechanical type in that the ramp-up is more controlled to provide a smoother transition from stopped to full speed. Other advantages include controlled ramp-down or "pump stop" features-which can reduce or eliminate water-hammer-a much smaller footprint and it is often less expensive. However, electromechanical reduced voltage starters are far more resilient when considering power fluctuations and temperature extremes and they tend to be easier to repair.
VFDs (variable frequency drives) are often used for constant level applications to provide a discharge to equal a varying inflow, reducing pump starts and stops. VFDs also have inherent "soft start/stop" capabilities via adjustable acceleration and deceleration times. Variable frequency drives may also be used to provide "phase conversion" in some applications. If only single phase power is available on site and the pumping requirements call for a motor horsepower that is impractical in a single phase configuration, a three phase motor may be used with the VFD providing phase conversion through the inverter circuit.
There are some important advantages when using VFDs instead of static or rotary phase converters, including controlled load ramp up/ramp down capability, simplified installation and smaller overall control system size. Some disadvantages of VFDs when compared to other phase conversion methods are the possibility of harmonics introduced on the power line and somewhat higher initial costs when dealing with lower horsepower applications.
Branch Circuit Protection
Depending on the installation, branch circuit protection for the load (motor) may be provided in the control panel, MCC (motor control center) or in a separate load center. Branch circuit protection will be in the form of a circuit breaker or fuses. Circuit breakers (for motor loads) are available in two types: thermal magnetic or motor circuit protectors.
Motor circuit protectors (also referred to as "instantaneous trip" or "magnetic only" circuit breakers) do not have the thermal characteristic and provide short circuit branch circuit protection only. Their adjustable magnet trip threshold reduces nuisance tripping when subjected to high inrush currents, and they often reduce costs by allowing the system integrator to stay with a smaller breaker frame size. MCPs must be used in conjunction with a NEMA rated starter (or other starter with a listed overload accepted by the MCP manufacturer) to maintain a UL listing.
Fuses provide equivalent, and in some cases, superior branch circuit protection when compared to circuit breakers in motor circuit applications. Fuses typically have a higher interrupt rating (often equal to or exceeding 200KAIC) and on average are more cost effective in situations where the voltage exceeds 250V. Fuses are not as well received by many operators in that they cannot be "reset" like a circuit breaker when tripped. They must be replaced, which may be inconvenient
A common misconception associated with branch circuit protection is that the fuse or breaker should protect a designated device. Circuit breakers or fuses do not protect individual components. They protect the other portions of the system and wiring by isolating the problem load. A circuit breaker for home kitchen outlets will not protect the toaster oven from shorting when a child tries to cook a Play-Doh pizza. The breaker will trip or the fuse will open and keep the short circuit situation from overloading the wiring and possibly causing a fire.
Level Control Method
There are many ways to sense the liquid level of a storage vessel. Choosing the best level control method for a specific water/wastewater application requires a comprehensive understanding of the system characteristics. The capacity and dimensions of the well or reservoir, how quickly it fills or empties, what type of liquid is being pumped and the location and type of pumping equipment installed are all determining factors for which method of level control is best suited for a particular application.
Liquid level sensing for pumping systems falls into two basic categories: point level (digital or on/off) sensors and continuous level (analog) sensors. Float switches, pressure switches, conductance electrodes/probes and capacitance switches are all examples of point level sensors. "Bubbler" systems may also fall into this category, depending on design.
A transducer is a device that converts one form of energy to another. A microphone is a transducer because it takes sound varying in amplitude and frequency and converts it to an electrical signal. The inverse of a microphone is a speaker. It takes an electrical signal and converts it to sound. The most widely used transducer types associated with pumping systems are pressure to electrical (submersible and inline), ultrasonic and radar. Some types of bubbler systems may also have a pressure-to- electrical transducer incorporated in the control package.
All level sensing devices, no matter the category, have advantages and disadvantages. Point level sensing devices (especially float switches for sump and lift stations) are still the most common and least expensive; however, they may require more maintenance due to buildup of debris on the sensors and the number of components involved to provide complete level control. If barriers to render the switching devices "intrinsically safe" are required, as in most sanitary sewer control applications, the cost advantage over continuous level monitoring is diminished significantly.
Transducers will likely be used in conjunction with a controller, which processes the analog signal to show the actual level and allows the operator to adjust the engage/disengage set-points for the pumps and alarms. Transducers have advantages over single point level sensors in that there is only one component to install in or on the storage vessel, reducing installation and maintenance time.
There are also advantages/disadvantages between transducer types. For instance, submersible transducers are simple to install, resistant to debris along the water line, very accurate and respond quickly to changing levels; however, they may have to be secured in some fashion to avoid being pulled in by the pump. An intrinsically safe barrier may also be required in some applications and in instances where there is a buildup of sludge; securing methods may have to be modified to prevent fouling.
Ultrasonic transducers are also accurate, inherently intrinsically safe since they are a "non contact" device and require virtually no maintenance when properly installed in a suitable environment. Some disadvantages are that they tend to respond somewhat slower than other transducer technologies, can be affected by moisture or frost forming on the lens, may experience false echoes from foam or steam and can be difficult to troubleshoot.
Radar level transducers have the same benefits as ultrasonic transducers and they are not affected by changing media or vapors, respond to changing levels quickly and are suitable for higher temperature ranges. However, they may be among the more expensive technologies and also tend to be difficult to troubleshoot.
Control Panel Location
The location of the pump controls are a crucial factor in designing a durable and serviceable system control package. The control enclosure must be suited for the environment with two basic ratings, indoor and outdoor. The enclosure considerations in this article are intended for non-hazardous locations.
Indoor enclosures have various NEMA ratings depending on the construction. NEMA Type 1 enclosures are designed to provide basic protection against contact with electrical equipment mounted inside the enclosure. They may not have continuously welded seams (for steel construction) or door gasket(s) and will have little resistance to dust, dirt and liquids. Most of the smaller enclosures will not have provisions for locking, but the larger NEMA Type 1 enclosures may.
NEMA Types 12 and 13 are also designed for indoor installations. They exceed the NEMA Type 1 rating in that they will provide protection from falling dirt or dust and falling or sprayed non-corrosive liquids. The overall construction includes continuously-welded seams when using steel or stainless steel, full gasketing on door(s), a robust door hinge and many door-latching options. Kits are available from most manufacturers for modifying NEMA Type 12 enclosures to make them suitable for outdoor use. This gives the design engineer more options when choosing the enclosure size and features. Other options include floor stand kits, interior lighting packages, fans/blowers, louvers/vents and enclosure temperature control equipment.
There are two basic types of outdoor enclosures, not taking intermittent immersion into consideration. These are NEMA Type 3R and NEMA Type 4. Both types are suited for outdoor use with NEMA Type 3R enclosures rated as weather resistant, and NEMA Type 4 rated as weatherproof. Though both enclosures are rated for outdoor use, they are also suitable for indoor applications without any modifications.
NEMA Type 3R standards do not require welded seams, door gaskets or multi-point door latching mechanisms; however, some manufacturers will rate their enclosures as Type 3R even though they include many of the Type 4 features.
NEMA Type 4 requirements include fully gasketed doors, sealed seams and positive latches securing the entire door perimeter. Both NEMA Type 4 and Type 3R enclosures can be fitted with inner door assemblies where an enclosure "dead front" is required for system security. Other options include custom painting, floor stands with various skirting configurations, viewing windows, lockable door latches, exterior door stops and heating and air-conditioning systems.
A NEMA Type 4X corrosion-resistant enclosure is often specified and is used as a default from many control manufactures. Type 4X enclosures have the same weatherproof rating as Type 4 with the only difference that the enclosure composition is non-ferrous. Materials such as fiberglass, varying grades of stainless steel, aluminum and plastic are used depending on the enclosure size and application. Recently, Underwriter's Laboratories announced that they are considering a revision to UL50 and UL508A to include NEMA Type 3RX (corrosion resistant N3R) enclosures.
Conclusion
There are many decisions to be made when designing a pump station control. The three topics discussed here represent areas that must be addressed to provide a durable, trouble-free pump control system.