Selecting Pump Class 125 and 250 Flange Connections

Before anything else, confirm the pump casing material, flange class, face style, and allowable working pressure from the pump nameplate, drawing, or manufacturer documentation. A cast marking such as 250 may identify a pressure rating rather than an ANSI/ASME B16.1 Class 250 drilling pattern. Do not order the mating flange until both the pressure basis and bolt geometry are known.

1. Pump flange identification

1. Record the pump manufacturer, model, casing material, nominal flange size, and service temperature.
2. Read every casting mark and nameplate value. Treat 125 or 250 as an identification clue, not a complete flange specification.
3. Place a straightedge across the sealing surface. A continuous surface in one plane is flat faced. A raised annular sealing area identifies a raised face.
4. Measure the flange outside diameter, bolt-hole count, bolt-hole diameter, and bolt-circle diameter. Compare those readings with the applicable dimensional standard or the pump manufacturer’s drawing.

Cast iron flanges and flanged fittings use ANSI/ASME B16.1 dimensions. Steel and stainless-steel flanges use ANSI/ASME B16.5, while ductile-iron flanges use ANSI/ASME B16.42. Material identification matters because the standards assign different class designations and pressure-temperature ratings.

Observed condition	Meaning	Next check
Cast-iron body with a flat sealing surface	Likely a flat-face cast-iron connection	Identify whether the drilling matches B16.1 Class 125 or Class 250
250 cast into an iron component	Could indicate a 250 PSI pressure rating rather than Class 250 geometry	Measure the bolt pattern and obtain the component documentation
Raised annular surface on the proposed mating flange	The flange is raised faced	Replace it with a compatible flat-face flange or obtain approval for machining
Bolt holes do not align freely	The class, nominal size, or drilling pattern is wrong	Stop; do not pull the joint together with bolts

2. Bolt-pattern decision

Use the measured drilling pattern to separate nominal pressure markings from dimensional class. ANSI/ASME B16.1 Class 125 and ANSI/ASME B16.5 or B16.42 Class 150 use the same bolting pattern for corresponding sizes, so those flanges can bolt together. Class 250 and Class 300 likewise share a bolting pattern. This dimensional compatibility does not make their materials, face styles, or pressure-temperature ratings interchangeable.

1. If the pump flange matches Class 125 drilling, select a mating flange with the corresponding Class 125/Class 150 bolt pattern. Then check its face style.
2. If the pump flange matches Class 250 drilling, select the corresponding Class 250/Class 300 pattern. Do not substitute a Class 125/Class 150 flange.
3. If the pump component is marked 250 but measures as Class 125 drilling, treat the marking as a possible pressure designation. Confirm the rating from the manufacturer’s drawing or the applicable AWWA documentation.
4. If any dimension conflicts with the expected pattern, stop ordering and assembly. Resolve the component standard and nominal size from certified documentation.

AWWA C110-rated cast-iron or ductile-iron fittings can create the most common identification trap. A fitting may carry a 250 marking for 250 PSI working pressure while retaining ANSI/ASME B16.1 Class 125 or ANSI/ASME B16.42 Class 150 geometry. It will not bolt to an ANSI/ASME B16.1 Class 250 flange, whose pattern is different from Class 125.

3. Pressure-temperature rating check

After confirming the drilling pattern, compare the maximum operating pressure and temperature with the rating table for the component’s actual material and standard. The nominal class number is not a universal pressure limit. Material and temperature change the permitted working pressure.

Component basis	Rating stated for that basis	Application limit
ANSI/ASME B16.1 Class 125	125 PSI at 353°F	Example rating at saturated-steam temperature
High-strength cast iron, B16.1 Class 125	200 PSI through 150°F	Use only when the specified material and temperature match
Ductile iron, B16.42 Class 150	250 PSI through 100°F	Does not establish the rating of a cast-iron pump casing
AWWA C110 high-strength cast iron with B16.1 Class 125 dimensions	250 PSI working pressure with at least a 3.0 safety factor	Apply only when the component is documented for this AWWA basis
B16.1 Class 250 cast-iron flange or fitting	300 to 500 PSI or higher	Exact rating depends on the applicable material and temperature table

Check the pump casing rating separately from the connecting flange. Standard flange geometry does not establish the pressure capacity of every valve or pump body because body shape, wall thickness, and material vary. The joint rating cannot exceed the lowest documented rating among the pump casing, both flanges, gasket, fasteners, and connected piping.

4. Mating-flange selection

For a flat-face cast-iron pump flange, specify a mating flange with the correct drilling and a flat sealing face. A factory-supplied flat-face flange is the direct selection because it avoids field uncertainty about machining, finish, and dimensional compliance.

1. Match the nominal pipe size and the measured Class 125/Class 150 or Class 250/Class 300 drilling family.
2. Select a material compatible with the piping specification and process fluid.
3. Specify a flat face where it mates to the cast-iron pump flange.
4. Verify the mating flange’s pressure-temperature rating against the maximum operating condition.
5. Obtain the pump manufacturer’s gasket and bolting requirements before assembly.

If only a raised-face steel or ductile-iron flange is available, removal of the raised face can produce the required flat mating surface. Have the flange supplier or a qualified fabricator confirm that the finished flange still meets the specified dimensional, surface-finish, material, and pressure requirements. Do not grind the face locally or leave an uneven transition.

Do not select Class 300 merely because the pump is marked Class 250. First verify the Class 250 drilling pattern. Likewise, do not interpret a higher mating-flange class as an increase in the pump’s allowable working pressure.

5. Face and gasket configuration

Use a flat/full-face gasket specified for the pump service between two aligned flat faces. The gasket material and thickness must suit the fluid, temperature, pressure, flange finish, and manufacturer’s bolting instructions. Read those values from the gasket data and pump documentation rather than substituting a generic sheet material.

Do not clamp a raised-face gasket between a raised-face steel flange and a flat-face cast-iron pump flange. That arrangement concentrates the bolt load near the bore while the outer portions of the cast-iron flange remain unsupported. Tightening can bend the cast-iron flange and initiate cracking. Removing the mating flange’s raised face and using the specified flat-face gasket distributes the clamping condition across the flat joint.

Joint configuration	Decision	Reason
Flat-face cast iron to compatible flat-face flange with specified flat/full-face gasket	Use	Correct face relationship and distributed support
Flat-face cast iron to raised-face flange with raised-face gasket	Reject	Concentrates load and can break the cast-iron flange
Flat-face cast iron to a machined-flat mating flange	Use after documented approval	Finished geometry, finish, and rating must remain compliant
Correct bolt pattern but unknown pressure basis	Hold	Bolt compatibility alone does not establish working pressure

6. Alignment and bolting checks

The pipe must meet the pump flange without force. Flange bolts align and clamp the joint; they must not correct pipe offset, angular error, or excessive flange gap. Pipe strain can distort the casing, damage the flange, and change pump alignment.

1. Support the piping independently and bring the mating flange into position with the pump isolated and depressurized.
2. Check that the faces are parallel and that the gasket contact surfaces are clean and undamaged.
3. Insert bolts through the holes by hand. Do not move on until every bolt passes without using it as a lever or jack.
4. Confirm that the gasket is centered and remains flat. Keep gasket material out of the flow passage.
5. Install the specified fasteners, washers, lubricant, and nuts. Use the pump or gasket manufacturer’s bolt-torque value; no universal torque follows from the class number alone.
6. Snug the joint in a cross pattern, then tighten in repeated cross-pattern passes to the specified final torque.
7. Recheck pipe support and pump alignment after tightening. A change indicates that the piping is loading the casing.

7. Resolving and verifying the connection

1. Document the pump flange as cast iron, flat faced, and either Class 125 or Class 250 drilling based on measured geometry and manufacturer data.
2. Procure the matching flat-face mating flange. For Class 125 drilling, a dimensionally compatible Class 150 flange may be used when its face, material, and pressure-temperature rating meet the design. For Class 250 drilling, evaluate the corresponding Class 300 pattern the same way.
3. If converting a raised-face flange to flat face, obtain written confirmation of the finished dimensions and rating from the flange supplier or fabricator.
4. Install the specified flat/full-face gasket, align the unsupported joint without bolt force, and tighten with the documented cross-pattern procedure and torque.
5. Pressurize the system using the project’s approved test procedure. Inspect the full flange perimeter for leakage and watch the joint for movement or distortion.
6. After reaching the operating pressure and temperature, inspect again and verify pump-to-driver alignment according to the pump instructions. Do not operate through leakage, a new alignment shift, or any sign of a cracked flange.

Frequently Asked Questions