1) Know what changes when the motive is steam or gas

Note that gas-motive eductors use a converging–diverging nozzle (to take advantage of gas compressibility), while liquid-motive units use a converging nozzle.

For evacuating/priming duty, the same three connections apply:

Motive supplies energy (through the nozzle).
Suction is evacuated by entrainment; the vessel pressure pushes more gas into the eductor.
Outlet discharges at an intermediate pressure; pressure recovery depends on motive-to-suction ratio and how much suction pressure (vacuum) is pulled.

2) Identify the right model family for evacuating / priming

Models for Prime Pumps and Evacuate Liquid Lines.

Steam-motive models shown:

SG (Steam): motive pressure range 30–150 psig, typical pressure recovery 15–20%, max suction lift -20 ft, minimum required NPSH 13 ft.
HG (Steam): motive pressure range 20–150 psig, typical pressure recovery 30–35%, max suction lift -20 ft, minimum required NPSH 13 ft.

3) Define the evacuating duty in a way you can size

For evacuating (pulling air/gas out of a line, casing, vessel), define:

A) Suction target

Starting pressure: usually atmospheric (14.7 psia)
Final pressure: your target vacuum (psia or inHg vacuum)
Gas temperature and composition

B) Evacuated volume + time (this is the key for “evacuating”)

Total volume to evacuate: V (ft³)
Required time: t (minutes)

Convert the job into an average standard flow requirement:

Important: evacuation is not constant-flow—capacity falls as vacuum deepens. So you size using the worst portion of the curve (deepest vacuum / highest backpressure combination).

4) Define outlet/backpressure correctly (include friction losses)

Two critical points for outlet sizing:

Include friction losses in the outlet line.
Calculate friction using the combined rate of motive + suction flows.
Keep the outlet line as large as or larger than the outlet connection.

For evacuating service, outlet/backpressure is often:

to atmosphere (0 psig) through a separator/silencer
to a header/scrubber/flare (positive psig possible)

Backpressure is often the #1 reason an ejector underperforms.

5) Select motive condition (steam or gas):

Our selection rule is:

choose the table motive pressure closest to or lower than your available motive pressure.

Apply that same rule to steam/gas motive:

Use flowing steam pressure at the ejector inlet (not a deadheaded gauge)
For steam: specify saturated vs superheated (steam quality affects performance)
For gas motive: specify gas type, pressure, and temperature

6) How to “pick the size” (the same sizing-factor logic, but with vacuum curves)

Our catalog sizes by:

finding a Tabulated Suction Flow
computing Desired Sizing Factor = Desired Suction Flow / Tabulated Suction Flow
selecting a unit whose tabulated sizing factor meets/exceeds the desired value
then calculating motive consumption from the tables

For evacuating gases with steam/gas motive, you do the same process, but the “tabulated suction flow” must come from the manufacturer’s gas/vacuum performance curve for the SG/HG (steam) or gas-motive ejector:

suction capacity vs suction pressure (vacuum)
at your discharge/backpressure
at your motive pressure

Adjustments for priming (startup / clearing air from liquid lines)

Priming and “evacuate liquid lines” are explicitly called out as steam-eductor applications in your catalog.
Priming is harder than steady gas evacuation because the suction stream often becomes two-phase (air + vapor + liquid slugs), and the load is highly transient.

1) Size for the “initial slug” load, not just steady leakage

Use two separate requirements and size to the larger:

A) Evacuation requirement (bulk volume):

remove the contained air volume in the casing/line in the allowed time

B) Ongoing requirement (leak/ingress + vapor):

any continuous air ingress, flashing, seal leakage, etc.

2) Apply a priming factor to the required suction capacity

A practical rule for priming/line evacuation (because capacity degrades with two-phase and pressure swings):

Priming factor: multiply calculated suction requirement by 1.5 to 2.0
- 1.5× when you expect mostly dry air and short lines
- 2.0× when you expect wet suction, long lines, or frequent liquid carryover

This aligns with the catalog’s general philosophy of correcting for real-world losses (it recommends using NPSH-based calculations to correct for temperature and friction losses for more accurate results).

3) Don’t ignore NPSH / suction conditions on steam-eductor priming

Your steam models show Minimum Required NPSH = 13 ft for SG/HG.
In priming service, if the suction side flashes or the line is hot, you can lose prime performance quickly—so use the more rigorous NPSH method the catalog points you to for temperature/friction corrections.

4) Installation tweaks that materially improve priming success

These are “make-or-break” for priming and are consistent with how we treat suction/outlet losses:

keep suction piping short/large; avoid high points that trap air
provide a small knockout/separator if you expect liquid carryover
size discharge generously and account for combined-flow friction

Any RFQ to us need to include (steam or gas motive, evacuating + priming)

Service: Evacuate/prime __________ (pump casing / suction line / vessel)
Volume to evacuate: ____ ft³
Time to achieve: ____ min
Start pressure: 14.7 psia (or ____ )
Final pressure: ____ psia (or ____ inHg vacuum)
Gas composition/temp: ____ @ ____ °F
Discharge/backpressure: ____ psia (includes downstream losses)
Motive: Steam / Air / N₂
Motive pressure (flowing): ____ psig, temp/quality: ____
Priming factor: 1.5× / 2.0× (state expected wet vs dry priming)
Requested output: performance curve, recommended model/size (SG/HG or gas-motive), motive consumption, and any staging recommendation.