How prime-assisted systems address the operational challenges unique to industrial facilitiesSeptember 16, 2021
The context of today’s industrial plant and facility operations dictates promoting worker safety, improving operational efficiency and reliability, and incorporating green practices — all of which translates to reducing costs. These demands are even making their way through to the “back of the plant” in the environmental management process.
Many industrial plants and commercial production facilities generate large volumes of wastewater and environmental runoff as part of daily operations. The wastewater and environmental runoff are collected in sumps, in‐ground pits or reservoirs, typically ranging in depths from 3 ft to 20 ft. When liquid levels within the sumps reach designated limits, the wastewater is transferred to storage, treatment or disposal.
For larger sump applications (minimum flows from 75 gpm and sumps with depths of at least 6 ft), complete pump stations (or lift stations) are generally applied for conveyance around the facility to the treatment system or discharge point. These pump stations can take different forms, based on the type of pump employed and the relative positioning of the system’s mechanical equipment. Most wastewater pumps are centrifugal but can vary from submersible systems (wet-pit) to above-grade, prime-assisted mechanisms. With each there are various characteristics that must be weighed by the facility, in the context of worker safety, operational efficiency and cost minimization.
Smith & Loveless addresses these issues with the introduction of EVERLAST pumping systems, a newly styled, packaged vacuum-primed pumping system approach that eliminates the typical confined space hazards of wet-pit, submersible approaches while improving the efficiencies gained with conventional prime-assisted methods.
Operator safety for industrial pump stations
In wastewater and storm water pumping, the major operations hazard (aside from the obvious combination of electricity and water-handling systems) is exposure to designated confined spaces, which in these cases would be the sumps and any associated below-grade valve vault. Designated confined spaces like wastewater sumps and submersible valve vaults typically require very deliberate maintenance safety procedures, proper training and additional personnel and gear to maintain proper safety protocol and prevent worker injury. Some facilities may choose to bring in outside service groups in order to maintain submersible pumping systems, but this strategy can drive up operating costs while still requiring some risk management of onsite personnel. Because most industrial wastewater submersible pumps yield service life of one to three years, requiring frequent rebuilds or replacement over a given period, industry is looking to other, safer alternatives for wastewater pumping.
It figures that if confined space hazards can be eliminated, then safety and ease of maintenance can be improved and costs can be eliminated. That is why the top-mounted, prime-assisted arrangements have emerged over time, including new EVERLAST vacuum-primed pumping systems by Smith & Loveless. These systems allow operators to immediately access all mechanical and electrical equipment at grade level — including the non-clog pumps, control panel and all valves — without the use of special safety gear or other precautions that usually go along with accessing confined spaces. Instead, the confined space hazards of the sump are essentially eliminated because access is no longer required, saving hundreds of hours of manpower and related maintenance, paperwork and labor costs.
This has been demonstrated in the municipal collection system market, where it has been documented repeatedly that above-grade pump stations reduce operation, maintenance and parts by more than 50 percent when compared to confined-space submersible pumping stations.
Priming from above – Self-priming or vacuum-priming?
Naturally, placing the pumping system above the sump, pit or wet well with the pump rotating assemblies above the sump liquid level — rather than submerging with submersible pumps — requires priming assistance. The liquid has to be lifted from the sump in order to “prime the pump” and commence pumping. There are two general ways that this occurs in centrifugal pump design: fluid suction-lift achieved with self-priming pumps and fluid suction-lift achieved with vacuum-priming.
Self-priming pumps are typically horizontal end-suction centrifugal pumps that “self-prime” by using internal recirculation within the pump to draw the liquid. After water fills the volute, the pump’s impeller will turn in a counterclockwise rotation. The initial prime of the pump is directed through an ever-increasing water channel into a discharge chamber inside the volute casing. There, the water and air separate. The heavy water falls back down into a recirculation port while the air is evacuated. While water recirculation occurs inside the casing and air is being released, low pressure is created at the eye of the pump impeller. The higher atmospheric pressure differential forces the sump liquid to rise up the suction pipe and pushes all of the air ahead of it into the volute casing where it is handled through the recirculation process. As the water arrives, the pump goes into complete operation. Overall, the priming stage can take several minutes from the beginning before full pumping commences.
Vacuum-primed pumping systems differ significantly from self-priming type pumps. First, where horizontally configured self-priming pumps are designed to prime and pump, vacuum-primed pumps are vertical, close-coupled pumps with oversized pump shafts and bearings. Rather than priming itself, these pumps rely upon a simple, ancillary priming system, comprising three basic components: a prime sensor, a solenoid valve and a separate 1/8 Hp vacuum pump. When the liquid level rises in the sump and tilts the low-level displacement switch or transducer, the vacuum pump is activated in order to lift the liquid into the volute casing. The vacuum pump turns off when the prime sensor senses sufficient liquid in the volute. The centrifugal pump activates once primed and commences pumping. From a totally non-primed condition, the system primes the pump within 60 seconds under standard rated conditions. Once the centrifugal pump is primed, it is designed to stay primed indefinitely.
Despite being separate from the actual centrifugal pump and requiring a separate vacuum pump, the vacuum-priming process is generally more efficient than self-priming because the self-priming pump motor must generate the priming action. For example, a 15 Hp self-priming pump may utilize 5 Hp simply for the priming cycle compared to just 1/8 Hp of the vacuum pump. Thus, the need for self-primers to both prime and pump increases the energy required to prime while lowering the overall operating efficiency. Compared to vacuum-primed pumps, the overall difference in wire-to-water efficiency can be as much as 20 to 25 percent, which can mean hundreds to thousands of dollars per pump station in annual savings when compared side-by-side.
In evaluation for prime-assisted pumping systems for wastewater and stormwater, whether vacuum-primed or self-priming, there are several things to consider: pump efficiency/power costs, operator safety, O&M time and cost including downtime, equipment durability and ease of access to pump internals. When undertaking an evaluation, planners should consider and explore answers to the following questions.
Pump efficiency/power costs
At normal operating conditions, what are the differences in wire-to-water efficiencies for the particular application between a self-priming pump and vacuum-primed pump? How much does that translate into annual power costs? Evaluating the application’s design points along the particular pump’s published pump curves can provide the data for calculating the pump efficiency and resulting power requirements in a given period of time. Improving pump efficiency can help lead to energy credits and LEED certifications for new construction.
Priming system nuances
What are the components that comprise the system’s priming scheme? What are the events that can prevent normal priming? How will typical priming time affect the sump/wet well size? For example, if one system takes several more minutes to prime than another, it can affect the sump volume capacity. It would need to be sized correctly to prevent potential overflows during heavy surges in flow.
What are the wearing parts that will have to be eventually replaced (at what costs)? Are there shims, v-belts (belt-drives) or wear plates that have to be adjusted? Seal maintenance: Does the pump require fresh-water or oil-filled seals (that require periodic checking and filling)? How many different kinds of valves are required for each system? When performing maintenance on these kinds of pumps, how easy is it to gain access to the volute and seal? These are important questions for assessing long-term O&M and life-cycle costs. Obviously, maintaining fewer wearing parts lowers labor time and associated costs. The better pumping system manufacturers will be able to provide real data on parts and maintenance labor.
What operator safety concerns apply beyond typical electrical shut-off procedures? For example, when accessing the volute, are there any issues with potential spillage or steaming concerns (generated from recirculation)? Specifiers and decision-makers should understand the differences in vertical and horizontal pump construction as it relates to maintenance. Overall, because confined-space requirements are eliminated, prime-assisted pump stations and systems do provide the safest approaches to wastewater and stormwater pumping systems for flows of at least 50 gpm compared to conventional submersible and chopper pumps.
Because prime-assisted pump stations and systems are above-grade, all of the equipment will be housed on a skid or steel base with different types of enclosure designs. Obviously, the location of the sump must be able to provide the vertical or horizontal access space in order to access the equipment. That said, there are no confined space entry requirements and valve-vaults to consider.
The key for today’s industrial pump stations centers on reducing hazards, maintenance and labor time, and overall costs. Taking the pumps out of the sump presents a cost-saving solution that safely addresses confined space restrictions while improving the operating efficiency and maintenance cost. The need to prime these above-grade systems represents a slight trade-off to the hidden submersible, but the safety and ease of maintenance benefits are clear. As above-grade pump stations continue to expand into industrial plants and facilities, decision makers should see that there are two primary types, which starts with how they are primed and how the pump is constructed. Understanding these differences will aid in planning wastewater and stormwater pumping projects in order to effect reliability, safety and an improved bottom-line.
About the author
Don Aholt offers more than 40 years of professional engineering experience in the wastewater management field. He serves as the vice president of the industrial division of Smith & Loveless Inc., a global leader in the engineering and manufacturing of packaged wastewater treatment and pumping systems. He received his bachelor’s in mechanical engineering and masters in sanitary engineering from the University of Missouri. He can be reached at firstname.lastname@example.org.