When should I change the tip seals on a dry scroll pump, and what happens if I wait?

Change them on hours, not on symptoms. Most manufacturers put the interval between 8,000 and 16,000 running hours on clean duty — Edwards nXDS is 16,000, XDS is around 8,000 to 10,000 depending on size. On dusty or vapour-laden duty, halve it. Waiting for a symptom is the expensive route, and this section explains why.

The tip seal is a filled-PTFE strip that runs along the crest of the orbiting and fixed scrolls. It wears continuously by design — that is how a dry pump seals without oil — and it wears into a fine white powder. There is no failure event: capacity and ultimate pressure fall off gradually, no noise changes, and the motor current barely moves. The pump keeps looking healthy while it stops doing its job.

The powder is the reason waiting costs more. As the seal thins, more of it ends up in the pump and in your line. It coats the bearings, loads the exhaust filter and, on the inlet side, travels back into whatever the pump was evacuating. A seal changed on schedule is a one-hour job with a kit; a seal run to the end can mean bearings, a cleaned pump body and a contaminated chamber.

The honest test, if you have no hour meter, is ultimate pressure with a blanked inlet on a gauge you trust. Compare with the manual figure. A pump that used to reach the low 10⁻² mbar range and now sits an order of magnitude higher, with no change in noise and no leak found, has worn tip seals until proven otherwise.

Two things to get right when you order. First, the kit is specific to the pump size and often to the build — the same model number can cover two generations with different seal cross-sections, so the serial number matters. Second, on a corrosive process specify the chemical-resistant variant; the standard elastomers in the kit are the part that fails first on solvent duty, not the PTFE itself.

Send the pump model, the serial number and the hours since the last change. For Edwards nXDS the kit is A735-01-801 (also written A73501801) and fits nXDS6i, nXDS10i, nXDS15i and nXDS20i; for XDS35i it is A735-02-983. We confirm against your serial before shipping.