How to Size an Eyewash Station: Flow Rate, Pipe Size and Supply Pressure

Short answer: Size an eyewash station from the flow requirement backwards. An eyewash needs 0.4 GPM at 30 psi at the outlet; a combination shower and eyewash needs around 20 GPM for the shower leg. Then check that the pipe, the pressure and the simultaneous demand can actually deliver it.

Step 1: Start from the required flow

ANSI/ISEA Z358.1-2014 sets 0.4 GPM (1.5 L/min) for eyewash and eye/face wash units, measured at 30 psi flowing pressure, sustained for 15 minutes. A shower delivers substantially more - commonly around 20 GPM - and a combination unit has to satisfy the shower figure while the eyewash runs.

Note the phrase "flowing pressure". A static reading at a closed valve tells you nothing useful. The number that matters is the pressure remaining while water is running.

Step 2: Check the pressure where the unit sits

A plant main at 60 psi can easily deliver 15 psi at a unit at the end of a long, undersized branch. Three things eat pressure:

• Pipe length and fittings - every elbow costs head

• Elevation - a unit on an upper floor or a mezzanine

• Competing demand - other fixtures open at the same time

Measure at the proposed outlet with the branch flowing, not at the nearest gauge on the main.

Step 3: Match the pipe to the duty

Sizing a combination unit on the eyewash figure is the single most common error we see in customer drawings. The eyewash is the smaller load; the shower sets the pipe size.

Step 4: Verify the 15-minute duration

Flow rate is a rate, not a volume. The supply has to sustain it for 15 minutes. On a plant main this is normally a non-issue; on a tank-fed or gravity system it is the binding constraint and must be calculated, not assumed.

Worked example

A laboratory building needs a combination shower and eyewash at the end of a 25 m branch. The main runs at 55 psi. The proposed branch is 1 inch, with six elbows and a 4 m rise.

Measured flowing pressure at the outlet: 34 psi. Shower flow achieved: within specification. Eyewash flow with the shower running: within specification. The installation passes because it was tested flowing, at the outlet, with both functions open - which is the only test that reflects an actual incident.

Frequently asked questions

Can I tee an eyewash off an existing washbasin line?

Often, for eyewash-only duty on a short run - but verify flowing pressure at the outlet first. Never tee a combination shower off a small domestic line.

Does a booster pump solve low pressure?

It can, but it adds a failure mode that depends on power. If you use one, make sure the eyewash still functions when the pump does not.

What flow do I need for an eye/face wash?

The same 0.4 GPM figure applies to eye/face wash units under ANSI, but the spray pattern is wider and must cover both eyes. Check the pattern, not just the number.

How do I size for a gravity-fed tank unit?

Work from tank volume: 15 minutes at the rated flow sets the minimum usable capacity, then add margin for fill level and head. A tank sized on a nominal figure without the 15-minute calculation will run dry.

Get the numbers before the purchase order

Send us the unit type, the branch length, the available flowing pressure and whether other fixtures run at the same time. We will confirm the pipe size and the flow you can expect, in writing, before the order is placed.

See the full range: eyewash stations, combination shower and eyewash, wall-mounted units.

Sources: ANSI/ISEA Z358.1-2014 flow and pressure values; CCOHS emergency shower and eyewash guidance.