# Design pressure & max working pressure

1. Oct 12, 2007

### wendyv007

hello!
Can anyone tell me what is the difference between max working pressure and design pressure of a boiler. how do we chose the max working pressure and design pressure if a boiler need to be designed.
thank you in advance

2. Oct 12, 2007

### FredGarvin

Depending on where you live, there will be design codes governing the allowables in boiler design. In the US that ASME's Boiler and Pressure Vessel codes. For any application, these codes must be followed.

3. Oct 12, 2007

### Q_Goest

In short, Boiler and Pressre Vessel (BPV) code defines the Maximum Allowable Working Pressure (MAWP) of a vessel to be the pressure that will create stresses in the shell that equal the allowable stresses given in the material section of the code. The piping code has similar criteria, but they call this the "Design Pressure".

The codes also require that relief devices be set no higher than the MAWP except for fire cases. It gets fairly complicated, but basically you need to have a relief valve set at the MAWP of the vessel. The pressure you operate this vessel at is then dictated by this relief valve setting. Most vessels are operated such that this relief valve is set no less than 10% above the operating pressure.

For example, if you need a vessel for a process that will operate at 100 psig, you shouldn't have a relief valve set at less than 110 psig which means you will need a pressure vessel with an MAWP of at least 110 psig.

4. Oct 12, 2007

### stewartcs

The design pressure is essentially the most severe condition of coincident pressure and temperature expected in normal operation.

The maximum allowable working pressure for a vessel is the maximum pressure permissible at the top of the vessel in its normal operating position at the designated coincident temperature specified for that pressure.

Source: 2007 ASME Boiler and Pressure Vessel Code (Division 1).

5. Oct 12, 2007

### stewartcs

You design the boiler based on the conditions you need it to operate. The material and design is based on whatever the desired pressure and temperature is for each case you are analyzing.

6. Oct 13, 2007

as per indian boiler regulation the max. w.p. of boiler is the design pressure. the inspecting authority can restrict the use of boiler to a pressure(which will be called max. w.p. of that boiler) less than design pressure.

7. Oct 19, 2007

### pr_anandh

i am working in piping design. i design one pipe and submit the detailes to my manager. i am using 90 degree elbow 1.25*D. my manager asked that derivation for elbow calculations. is there any of them know it give me the details for me. and then suggest any book also. veryyy urgent.

8. Oct 19, 2007

### Q_Goest

Hi pr andandh,
Regarding the pressure rating of elbows and similar fittings;
- If you’re using standard fittings (purchased, off the shelf) then they should be manufactured to a given standard. In the US for example, pipe fittings (including elbows) are fabricated per ASME B16.5 and B16.9. Wrought copper fittings are per ASME B16.22. Other specifications exist for other types of fittings.

All specifications for fittings will provide guidance on pressure ratings. So if you are using standard fittings, you can find the pressure ratings of them using those standards.

- If you’re simply bending a piece of pipe or tube, there are two different ways I’ve seen used to evaluate this. The issue with bending a tube or pipe is that the conduit wall is stretching on the outside radius and compressing on the inner radius of the bend. The result is a thinning on the outer radius and thickening on the inner radius of the bend. Because of this thinning, there should be a decrease in working pressure at the bend. But because of cold working and the resulting toroidal shape, the working pressure should increase at the bend. Depending on how you interpret different codes and how conservative you want to be, there are two distinct view points I’ve seen:

1. Disregard the thinning because of the benefits, and rate the bend at the same pressure as the rest of the pipe or tube. Pressure testing I’ve seen has largely confirms this affect, but this particular view isn’t conservative.
2. Rate the bent tube or pipe at the pressure rating an equivalent piece of straight tube or pipe would have given the reduction in wall thickness. You can determine the reduction in wall thickness simply by doing a geometric analysis of the bend, remembering that the conduit is bending around it’s neutral axis. If you really can’t derive this wall thinning yourself, I’ll provide, but you should be able to do this without too much effort.