A press joint is permanent. That's the point of it, and it's also the thing to plan around: anything in a stainless press line that will ever need to come out (a pump, meter, filter, PRV, strainer or valve) needs a break point, or the next person has to cut the pipe. In the Standard Press 316 stainless range that break point is a union up to 54mm, or a flange above it.
The three unions
- Barrel union: press x press, a breakable joint in a straight run. 15mm to 54mm.
- Male BSP union: press x male thread, with the union break built in. 15mm x 1/2" to 54mm x 2".
- Female BSP union: press x female thread, with the union break built in. 15mm x 1/2" to 54mm x 2".
Worked example: a pump with 1" female ports on a 28mm line
Option A: two male BSP unions, 28mm x 1"
2 x $83.03 = $166.06. Two press joints, two threaded joints, done.
Option B: two male BSP adaptors plus two barrel unions
2 x $29.37 (adaptors) + 2 x $56.14 (barrel unions) = $171.02. Six press joints, two threaded joints, plus tube offcuts between them.
The BSP union is $4.96 cheaper, four press joints fewer and shorter. When the union connects straight onto threaded equipment, use the BSP union.
Where the barrel union wins
The barrel union is for breaks in a pressed run, where there's no thread to connect to:
- Either side of a press-ended item, like the press ball valve. A press valve is permanent; a barrel union each side lets you lift the valve out with its short spools.
- To split a long prefabricated run into sections you can install, or remove, in pieces.
- Where a pipe spool may need to come out for access to something behind it.
Prices: 15mm $29.19, 22mm $40.33, 28mm $56.14, 35mm $68.05, 42mm $84.54, 54mm $119.97.
Male or female BSP union?
Match the equipment: a pump or meter with female ports takes the male union; a valve or fitting with a male thread takes the female union. Common pairings run 15mm x 1/2", 22mm x 3/4", 28mm x 1", 35mm x 1-1/4", 42mm x 1-1/2" and 54mm x 2", with a few step sizes such as 15mm x 3/4" and 22mm x 1". At 22mm x 3/4", the male union is $59.68 and the female $65.54.
Above 54mm: use flanges
The unions stop at 54mm. For 66.7mm and up, the equivalent break is a flanged joint, using the Table E or ANSI 150 flange adaptors.
Union tips
- Union joints seal on a flat gasket. For lines carrying oils, fuels or high-temperature media, a green FKM union gasket is listed in the range (not yet available to order online; ask PSA). See stainless press seals.
- Put an isolation valve on the system side of each union, so the equipment can be isolated before the union is cracked. See press vs threaded ball valves.
- Leave spanner room. A union nobody can get a spanner on isn't a break point.
- Support both sides so the pipework doesn't drop when the union is undone. See supports for stainless press.
Frequently Asked Questions
What's the difference between a barrel union and a BSP union?
A barrel union is press on both ends and creates a break in a pressed run. A BSP union is press on one end and threaded on the other, connecting straight to threaded equipment with the break built in.
What sizes do stainless press unions come in?
15mm to 54mm for the barrel union and the male and female BSP unions. Above 54mm, use flange adaptors as the break point.
Do I need a union next to a pump on stainless press?
If the pump will ever be removed, yes, or use a flange. Press joints are permanent, so without a break the pipework has to be cut.
Which is cheaper for connecting a pump: BSP unions or adaptors plus barrel unions?
BSP unions. On a 28mm line with 1 inch ports, two male BSP unions cost $166.06 against $171.02 for adaptors plus barrel unions, with four fewer press joints.