Buttweld Class 3000 Unions

Buttweld Unions / Buttweld Class 3000 Unions
Hart Buttweld O-Ring Union

Fail Safe Leakproof Reliability

At any pressure, HART O-Rings actually strengthen the seal rather than loosen it.

VViton® Florocarbon A (FKM-A)
EEthylene Propylene (EPDM)
ESHigh-Temp Steam EPDM [up to 550*F max] (EPDM E0962)
TTeflon® Tetrafluoroethylene (PTFE)
BNitrile / Buna N (NBR)
NNeoprene® Polycloroprene (CR)
KKalrez® 4079 Perfluoroelastomer (FFKM)
HNHydrogenated Nitrile (HNBR)
EPNSF-61 Approved EPDM (Ethylene Propylene)
GMetallic Graphite Spiral Wound Seal
INo O-Ring (Integral Seat "Ball to Cone")
Teflon®, Viton®, Kalrez® are
Registered Trademarks from DuPont


The O-ring is fitted into a machined groove between the two halves of the union. When the halves are drawn together the O-ring compresses to an ovalized cross-section, forming a positive, resilient seal which serves to block the fluid, thus sealing even at low or no pressure.

Under Load

As pressure is increased the O-ring is forced to flow and is "squeezed" into the downstream side causing the O-ring to conform to the shape of the end, blocking the groove gap. The more the system pressure increases, the more effective the seal.

Straight-Away Breakout

The flat faced construction allows for slipping the component in and out without disturbing the surrounding tube or pipe sections. Pipe alignment is much easier with Hart Unions since it is unnecessary to spring the line during make-up or disassembly.

Specialty Orders

Specialty O-Rings also available. Inquire within.


The chart below contains the necessary information to place an order. Use the appropriate code that corresponds to your application. When a size reduction or change in material is needed, simply use a slash ("/") between the sizes or materials.

Please reference the two examples below which include descriptions of the unions and their corresponding HART Part Numbers.

O Orifice:
Flat Orifice Plate with Double O-Ring Seal
D Dielectric (Insulating):
Dielectric Coating applied to union tailpiece
H Hammer Nut:
3-Lug Hammer Blow / Lug Nut Option
Female NPT (FNPT) Class 3000 Pipe 31 31 0  (1/8")
1  (1/4")
2  (3/8")
3  (1/2")
4  (3/4")
5  (1")
6  (1 1/4")
7  (1 1/2")
8  (2")
9  (2 1/2")
10 (3")
11 (3 1/2")
12 (4")

V Viton® Fluorocarbon A (FKM-A)
E Ethylene Propylene (EPDM)
ES High-Temp Steam EPDM [up to 550*F max] (EPDM E0962)
T Teflon® Tetrafluoroethylene (PTFE)
B Buna-N (Nitrile)
N Neoprene® Polychloroprene (CR)
K Kalrez® 4079 Perfluoroelastomer (FFKM)
HN Hydrogenated Nitrile (HNBR)
EP NSF-61 Approved Ethylene Propylene (EPDM)
G Metallic Spiral Wound Graphite Seal
I No O-Ring (Integral Seat "Ball and Cone")
Viton®, Teflon®, Neoprene®, and Kalrez® are registered trademarks of their respective owners.
A Aluminum 6061
B Brass 360 or 464
CS A105/A350-LF2, A105 Normalized, and 12L14 Carbon Steel
304L A182F304/304L Stainless Steel
316L A182F316/316L Stainless Steel
73 70/30 Copper Nickel
91 90/10 Copper Nickel
H Hastelloy C276
I Inconel 600
M Monel 400
T Titanium T-2
A20 Alloy 20 (Carpenter Steel C20)
Class 3000 Pipe
32 32
Socket Weld (SW)
Class 3000 Pipe
33 33
Socket Weld (SW)
Class 3000 Tube
34 34
Butt Weld (BW)
Class 3000 Pipe
35 35
Copper Sweat (SWEAT)
Class 3000 Tube
36 36
Female NPT (FNPT)
Class 6000 Pipe
61 61
Class 6000 Pipe
62 62
Socket Weld (SW)
Class 6000 Pipe
63 63
Socket Weld (SW)
Class 6000 Tube
64 64
Butt Weld (BW)
Class 6000 Pipe
65 65
Male NPT (MNPT), Socket Weld (SW), and Butt Weld (BW):
Place desired Wall Thickness in parentheses. Note: S40 Standard for Class 3000 and S80 Standard for Class 6000 unless otherwise specified.
Orifice Unions: Place desired drilled orifice bore in parentheses. For blank plate, omit the bore or note "(blank)".


BSPT-F BSPT (British Standard Pipe Taper) - Female, Rc
BSPT-M BSPT (British Standard Pipe Taper) - Male, R
BW TUBE Butt Weld with Tube O.D.
MALE PIPE END Male Pipe End (unthreaded) for socket welding.
ACME Nut ACME Thread Nut for specialty applications


Example 1: a Class 3000, 1" Female NPT x 1" Female NPT, Viton O-Ring, Stainless Steel 316 connections:

Example 2: A Dielectric Class 3000 Union, 1" Male NPT x 3/4" Copper Sweat, EPDM O-Ring, Stainless Steel 304L Tailpiece with dielectric coating, Lead-free Brass 464 Threadpiece:

For more information, a quote, or to order, call 1-800-769-0503 or contact us at sales@hartindustries.com

3535 Series

Butt Weld, Class 3000 Unions

HART Industrial O-Ring Unions are engineered to provide Class 3000 Service and are the most popular union in the industry for general purpose applications. The flat face design provides turbulence-free fit across seats. This design is ideal where piping requirements dictate the necessity of having a flat face seal to make and break the pipe line. All HART Unions are made in the USA!

Standard O-Ring Union Features:

  • Turbulence Free: All unions provide a turbulence-free fit across the seats
  • O-Ring Isolation: The O-Ring seal is strategically located in the thread piece face reducing O-Ring media contact and added protection against abrasives and erosion.
  • Precision Machined components: All unions are precision machined to provide high quality and fail-safe leak proof reliability.
  • Interchangeable End Connections: Threaded, socket weld, and butt-weld ends are interchangeable. This feature reduces overall costs by eliminating unnecessary nipples, bushings, couplings and inserts.
  • Material Versatility: Unions can be provided in all standard metals or combination of metals. This includes 304 Stainless, 316 Stainless, A 105 Carbon Steel, Brass, Monel 400, Hastalloy, and Titanium.
  • Excellent vibration resistance: Seals will not loosen, even under extreme pressure and pressure surges.

3D Animations


3535-1 1/4" 2.63 1.50 0.75 1.28 1.33 0.33
3535-2 3/8" 2.87 1.69 0.87 1.41 1.44 0.4
3535-3 1/2" 2.93 1.87 1.06 1.42 1.49 0.7
3535-4 3/4" 3.06 2.20 1.25 1.46 1.58 1
3535-5 1" 3.09 2.62 1.62 1.49 1.58 1.75
3535-6 1-1/4" 3.27 3.06 1.87 1.61 1.62 2.5
3535-7 1-1/2" 3.30 3.37 2.12 1.65 1.62 3.5
3535-8 2" 3.52 4.12 2.75 1.71 1.78 4.25
3535-9 2-1/2" 3.62 5.00 3.25 1.78 1.81 6
3535-10 3" 3.68 5.62 4.00 1.84 1.81 8