Primes Five Times as Fast — The S&L system normally primes the pump within 30 seconds or less, while the self-primer can take up to three to five minutes. The self-primer’s longer priming times require larger wet wells in order to hold the wastewater while the pump is priming. To achieve shorter priming times, the S&L pump uses a vacuum pump capable of handling large quantities of air. Note that depending on RPM, total dynamic suction lift, elevation, suction pipe dia. etc., self-primers typically can actually prime within 60 seconds for most applications. However, it is true that vacuum priming, on average, is a much faster process. Other self-primer manufacturers, for example, will approve designs as long as they do not exceed five minute re-prime time so the five minute statement can be true.
Lower Horsepower to Prime
Fractional Horsepower Pump — The S&L vacuum pump is equipped with a 1/6 or 1/2 Hp motor depending on suction pipe size. The selfpriming station uses the main pump motor, and is operating at the equivalent of “deadhead” conditions. A 15 Hp pump can use as much as 5 Hp during the priming operation. Thus, S&L pumps use less horsepower and prime much faster.
Fewer Valves Required
No Suction Flap Valve or Air Release Valve — Unlike self-priming designs, S&L pumps do not require air release valves or suction flap valves. If the air release valve fails, the pump may not prime. The flap valve could jam open, wear out, could be missing, or a suction flange might be pitted and not allow the flap valve to seat properly causing frequent loss of prime. The air release valve or the valve discharge line has a tendency to clog, or the valve may fail. The flapper valve is considered an energy savings device, however, regular maintenance and replacement is required to ensure proper seating. Self-primers are designed to re-prime regardless of the existence or condition of the flapper valve as long as the volute is half full. If the air relief valve becomes clogged or fails, the pumps will fail to prime if the discharge pressure is > 7 PSI. Self-primers can typically compress and expel air in the suction line/pump as long the discharge pressure is < 7 PSI. The simpler S&L system does not present these problems.
Simple Priming System
S&L Priming Advantage No. 1: Reducing Static Suction Lift — For an S&L wet well mounted pump station, static suction lift is calculated from the pump (off) elevation to the centerline of the pump volute elevation. The centerline of an S&L pump volute is approximately 6”- 8” above grade. For a self-priming station, static suction lift is calculated from the pump (off) elevation to the center line of the pump inlet suction elevation. The centerline of the pump suction for a typical self-priming station is 23”- 28” above grade. This saves nearly two feet of total dynamic suction lift, resulting in faster prime times, energy savings, and system design advantages.
S&L Priming Advantage No. 2: Simplifying Design — The S&L priming system is independent from the pump’s mechanical performance. A self-priming pump’s suction lift capability is, on the contrary, dependent on the pump shaft speed for low RPM applications. For example, if the hydraulic condition requires a shaft speed of 850 RPM, the maximum total dynamic suction lift for a 6” self-priming pump is 12’. A 6” S&L pump primes up to 20’ at the same hydraulic condition and shaft speed because the priming system is independent from the pump performance.