The pumped liquid or liquid-gas mixture first enters the impeller cells and the side channel through the suction opening. The side channel does not extend over the full circumference here, but is interrupted at one point. Due to the rotary motion of the impeller and the centrifugal effect, the pumped liquid has to circulate back and forth several times between the cells of the star-shaped impeller and the side channel, while a circular flow direction is superimposed by the rotation of the impeller. This results in the typical circular-helix flow lines on which the pumped liquid moves through the side channel stages. Due to the multiple interaction of the pumped liquid with the impeller, a very intensive energy transfer occurs.
With side channel pumps, delivery heads (i.e. pressure increases) can be achieved which are 8 to 9 times higher than those of conventional centrifugal pump impellers rotating at the same circumferential speed - with the same impeller diameter. At the end of the side channel, the pumped liquid is led into the discharge port and into the next stage or the discharge nozzle of the pump. A partial flow is led back to the suction side through the interrupter, in which the radial and axial operating gaps are minimized. The impeller blades passing through the area of the interrupter act here as a dynamic seal between the discharge and suction sides of the side channel stage. The minimum operating speed should not fall below 900min-1.
NPSH and inflow ratios of the Side Channel Pump
The NPSH (Net Positive Suction Head) is also referred to as the holding pressure head and can be used to evaluate the delivery and, in particular, the suction behavior of a pump. It is not a measure of the self-priming capability, but describes the minimum inlet head above the saturation pressure of the pumped medium. In other words, it is the smallest necessary overpressure that must be present at the suction nozzle so that a (nearly boiling) liquid does not evaporate. The NPSH is therefore a measure of the cavitation tendency of a pump.
If the NPSHA value of a system falls below the required NPSHR value of a pump at its suction nozzle or impeller inlet, cavitation (formation of vapor bubbles) will occur due to local pressure reduction and, as a consequence, a reduction of the delivery head.
Pumps with a low NPSHR can therefore transport media closer to the vapor pressure curve than those with high NPSHR values. In particular, pressurized liquefied gases or vapor condensates can be conveyed extremely cost-effectively and safely with S-series side channel pumps (SRZS, SEMA-S) and their low holding pressure heads.
Auxiliary systems for cooling these special media or complex multi-level installations for generating large plant-side inlet heights are not necessary. In addition to the media properties, the following are decisive for determining the NPSHA value of a system
- Vapor pressure pD at conveying temperature and
- - density ρ
and the feed ratios
- Inlet height HZ geo and
- pipe losses HV
mainly the absolute static pressure at the impeller inlet, which is composed of
- atmospheric pressure pA and the
- - static pressure in the vessel pB (rel. to pA)
At each operating point, the pump NPSH must be less than the plant NPSH to ensure trouble-free continuous operation. It is usual to include a safety margin of 0.5m.