Main Drains: the Deep End of Pool Hydraulics

ABS main drain for liner pools by Waterstar Egypt: 170 mm suction grill, 2 inch thread, gaskets and flange included. Safe twin-suction design advice from the factory.

What the main drain actually does

The main drain sits at the deepest point of the pool and gives the pump direct access to the bottom layer of water — the coldest, heaviest layer, where sunk debris, sand and precipitated chemicals collect. While the skimmer polishes the surface, the drain turns over the deep water that would otherwise stagnate, which matters even more in heated pools and through long winterising periods. It is also the only practical way to empty the pool completely for repair or refinishing. In a well-balanced installation the drain carries a substantial share of the suction flow, with the rest taken by the skimmer line, and both meet at the pump through separate valves so the operator can shift the balance at will.

MD-1-L: the Waterstar main drain for liner pools

For liner pools Waterstar manufactures the MD-1-L, a round ABS main drain with a suction grill of 170 mm diameter and an inside hose thread of 2 inch. The generous grill area is deliberate: spreading the suction over a wide surface keeps water velocity at the cover low, which protects swimmers and stops the drain from pinning debris against itself. The body ships complete with gaskets, screws and a flange — the parts that matter most on a liner pool, because the flange clamps the membrane between machined faces and the gasket seals the penetration permanently watertight. ABS shrugs off pool chemicals and UV, so the drain lasts as long as the shell it is cast into.

Two suction openings, always: the entrapment rule

One rule outranks every other on the suction side: never let a single main drain be the only opening on a pump line. If a body blocks a lone drain, the full suction of the pump concentrates on that one point — this is how entrapment accidents happen. Standard practice, and ours, is two drains spaced well apart on one line, or a drain balanced with the skimmer on the same pump, so that blocking one opening simply shifts flow to the other. The wide 170 mm grill of the MD-1-L works in the same direction, keeping face velocity low and breaking up the vortex that would otherwise form above the outlet. Design it in from day one; it costs one extra fitting.

Placement, and how to get the whole scheme right

Place the drain at the true low point of the floor, with the slab or liner base sloping gently toward it, and keep it clear of the walls so the grill draws evenly from all sides. Run its line to the pump through its own valve, never teed blindly into the skimmer pipe, so the two suctions can be balanced during commissioning. The drain is only one node in the circuit — its size must agree with the pipework, the pump and the filter, and Waterstar manufactures all of them in the same Egyptian factory. Send us your pool dimensions and depth profile and our engineers will return a complete hydraulic scheme, from drain and inlets to pump, filter and skimmer.