On Special Assignment Side-Channel Pumps — Real Problem Solvers When You Need Something Special
Side-channel pumps are the ideal choice in applications where self-priming, gas and vapor handling, low NPSH and low noise emissions are major design criteria.
The physical properties of the fluids you are handling often make it even tougher to select the right pump. Pumps installed at storage facilities for liquefied gas or other low boiling fluids are a good example. The pumps are usually placed in a central location to simplify installation. As a result, suction/supply lines tend to be long, and the pipes are exposed to sunlight. The heating effect combined with low head (at least when the storage tanks are nearly empty) often causes vaporization/outgassing in the pipes. In situations like this, the pumps must be designed to handle large volumes of steam or gas. Side-channel pumps are an obvious solution at flow rates up to 35 m³/h. Because of the way they are designed, side-channel pumps generate high pressure at relatively low flow rates. The flow rate can be regulated with greater precision, because the characteristic curve (see Fig. 2) has a steep slope. Stable pump operation can be maintained in the permissible portion of the curve, and users can avoid running the pump below the minimum flow rate.
At tank farms, it is particularly difficult to position the tanks so that there is sufficient suction head to ensure adequate NPSHA. The situation is similar with distillation columns, where the medium being fed into the discharge pumps is usually near the boiling point.
Safety regulations require that tanks or tank trucks should be unloaded from the top, which places extra demands on the discharge pumps. Pumps used to unload tank trucks have to meet stringent suction and NPSHR requirements. To comply with safety regulations, the diameter of the drainage nozzle on tank trucks must not exceed DN80, which means that flow resistance in the suction line is relatively high and NPSHA is very low. Pump NPSHR must also be low.
Side-channel pumps with low NPSH are available. These pumps have an axial inlet and an NPSH impeller as the first stage on the suction side. The impeller is a closed radial impeller with an extra large suction opening. The flow rates are relatively low, and the axial inlet prevents pressure loss. With this design, very low NPSHA (<0.5 m) is achievable. To create the best possible flow conditions, the calming section in front of the pump (a straight piece of pipe where the flow is not disturbed) is very long (see Fig. 1).
Combination pumps can generate even higher output. A supplemental side-channel stage runs on the shaft in parallel with the main centrifugal pump hydraulics (Fig. 3). Entrained gas or steam can flow through the side-channel stage. As is the case with side-channel pumps, these pumps can vent the suction line if there is sufficient fluid in the pump. These pumps, which are normally used to handle fuel and liquefied gas, are suitable for volume flows up to 350 m³/h.
Stringent safety requirements
Especially on applications with low-boiling fluids, it is important to ensure that the heat generated by the pump is kept to a minimum. The need to comply with stringent emission control regulations is driving demand for magnetic coupling technology in hydrocarbon applications. The following effect has been observed with butadiene at a tank farm. Butadiene is pumped from a tank to the process. Two similar chemical process pumps with magnetic couplings were installed. Each pump had a 2-pole drive and metallic can. Eddy current loss resulted in heat generation. Pump 2 had a somewhat more powerful magnetic coupling than pump 1, and the can introduced a bit more heat into the pump. Pump 1 ran without problems, but pump 2 went down after a short period of time due to a plain bearing failure. Failure analysis revealed that the bearing fault was caused by insufficient lubrication. The additional heat in pump 2 caused the butadiene to vaporize, blocking the flow of lubricant to the bearing.
There are a number of ways of avoiding this type of problem on systems with magnetic couplings.
- Non-conductive cans, made for example of ceramics (ZrO2).
- Slower rotational speeds (4-pole) which reduce eddy current losses to a quarter of the level in a pump with 2-pole drive.
- Side-channel pumps with a 4-pole drive, which are designed to handle media containing entrained gas or steam. There are a number of machines with axial inlet and very low NPSH.
Side-channel pumps with segmented housings have a number of seals where leakage could occur. Barrel housings on current designs eliminate this disadvantage (Fig. 4). One process pump with a PN 40 nominal pressure rating is designed for liquefied gas, low-boiling fluids and condensate. The use of a barrel housing makes it easier to design pumps with higher pressure ratings. Only the barrel housing, can and mounting need to be modified. This type of design reaches its limits if the can walls have to be very thick to contain the pressure, and eddy currents rise to unacceptable levels. Depending on the drive power rating, it is possible to design systems with magnetic couplings which are rated at up to 100 bar.
On side-channel pumps which are used to handle flammable fluids and which provide self-venting on the suction line as long as there is a sufficient level of fluid in the pump, the question is whether the interior of the pump is Atex compliant. When looking at this issue, it can be helpful to see the similarities with the safety features of a liquid ring vacuum pump. The suction process in a side-channel pump works on the same principle. Liquid which is filled into the pump forms a ring which together with the vaned impeller creates cells near the hub which transport the gas/air.
Ensuring proper circulation of the fluid in a liquid ring vacuum pump provides explosion protection. When redundant monitoring systems are deployed, it is possible to achieve compliance with Category II 1 G on the pump interior based on type testing which has already been carried out. Similarly, if the side-channel pump is filled sufficiently, the mixture in the pump is so „rich“ that there is no possibility of an explosion. Assuming that the fluid level in the pump is monitored, self-priming side-channel pumps can be safely operated in explosion hazard zones.
Side-channel pumps are normally supplied with a Category Ex II 2 G Certificate of Conformity, which means that they are suitable for Zone 1, and that includes the pump interior.
Because gas/air is only transported briefly during the start up phase in a side-channel pump, the Zone 0 definition does not apply. (The Zone 0 definition applies if an explosive mixture is present continually or for an extended period of time.) The Category Ex II 2 G side-channel pump mentioned above ensures safe, compliant operation.
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