Maximum Allowable Hydrotest Pressure For Piping

Abhijit.S.Musale

www.abhijitsmusale.com | October 2024

Keywords

Piping
Hydrotest
Pressure Test
Maximum Allowable Pressure

Summary

Hydrostatic testing is a critical procedure in piping engineering, ensuring system integrity and compliance with safety codes. While the conventional practice is to test piping systems at 1.5 times the design pressure, the ASME B31 code stipulates both minimum and maximum limits for hydrotest pressures. The minimum requirement is 1.5 times the design pressure, whereas the maximum is constrained by the condition that induced stresses must not exceed 90% of the material’s minimum specified yield strength (MSYS). This article provides an example to calculate maximum allowable stress for piping with hypothetical input data showing that a pipe designed for 48 bar can be safely tested up to 88.42 bar, though the conventional 72 bar (1.5× design pressure) remains within safe limits. The example highlights the importance of balancing code requirements with material properties to avoid over-stressing during hydrotests, thereby ensuring both safety and compliance in piping system validation.

Introduction

For experienced piping engineers, the answer often comes quickly: the hydrostatic test pressure should be 1.5 times the design pressure of the piping system. In practice, we simply multiply the design pressure by 1.5 to determine the required test pressure.

However, the situation can be more complex. For example, when multiple piping systems with different design pressures are tested together, the system with the highest design pressure dictates the hydro test pressure. As a result, some of the lower-pressure systems may be subjected to more than 1.5 times their own design pressure.

According to the ASME B31 code, the hydrostatic test pressure for a piping system must be at least 1.5 times the design pressure. This value represents the minimum allowable test pressure. But what about the maximum? The code also specifies that the test pressure must not exceed the level at which the stress in the pipe material reaches 90% of its yield strength. Each material has a defined Minimum Specified Yield Strength (MSYS), and this property sets the upper limit for safe hydro testing.

Lets understand this with one example. Suppose a piping has design pressure of 48 bars and design temperature is 135 Deg C. Based on velocity criteria and volumetric flow rate, selected pipe size is DN 250 which has OD 273.05mm. Lets consider corrosion allowance 1.6mm and mil negative tolerance 12.5%. The pipe material is ASTM A106 Gr.B, then what will be the maximum possible pressure at which the pipe can be tested? Or maximum allowable pressure up to which the pipe can be pressurised.

First, the minimum wall thickness required need to be calculated with formula given in the ASME B31.1 section 104.1.2. The same formula is shown below.

t_m = P. D_o 2(S. E. W + P . y) + A

(1)

Where,

t_m = Calculated minimum wall thickness required.

D_o = Outer diameter of the pipe. As per standard pipe sizes from ASME B36.10.

S = Allowable stress according to mentioned in the mandatory appendix A of ASME B31.1. Allowable stress is temperature dependent property. Higher the temperature of the material the lesser the value of allowable stress. This value is usually 50% to 60% of the minimum specified yield stress.

E = Weld quality factor. For seamless pipes it is considered as 1. it is applicable to longitudinal seam welded or spiral welded pipes. It basically reduces the allowable strength according to the type of welding used to fabricate the pipe. This factor is different then W. Unlike W this factor does NOT depend on temperature. in B31.1 its value is included in the allowable stresses mentioned in Appendix A, Where as, in B31.3 its value has to be taken from table A-1B and has to be multiplied with allowable stress.

W = Weld strength reduction factor. It is temperature dependent factor. As temperature of the material goes higher the value of this factor reduces. It is defined in the table 102.4.7 of ASME B31.1. This factor is considered because at higher temperature, weld joint's creep rupture strength gets lower then the base metal's.

y = It is a co-efficient defined by table 104.1.2 of ASME B31.1. Its value is about 0.4 for ferritic and austenitic streel at less then 480 Deg C. with higher temperature its value increases. The value of y is also sometimes taken as zero to get conservative thickness value.

A = It is additional thickness allowance in mm. It is given to accommodate the need to make threads on the pipe, give it mechanical strength or for corrosion allowance. Usually this value is taken 1mm or 1.6mm.

Following are given parameters,
Design Pressure (P) = 48 bars = 4800000 N/m²
Design Temperature (T) = 135 ^oC = 408K
PIpe Size Selected (DN) = 250DN
Pipe outer diameter (D_o) = 273.05mm
Pipe Material = ASME A106 Gr.B
Corrosion Allowance = 1.6mm
Mill Negative Tolerance = 12.5%

Allowable stress for A106 material at 135 Deg C according to ASME B31.1 mandatory appendix A shall be 17.1 ksi (117931034.5 Pa).

t_m = 4800000 x 0.27305 2 x ((117931034.5 x 1 x 1) + (4800000 x 0.4)) + 1.6 1000

(2)

t_m = 0.007067788 m = 7.06 mm

Minimum required thickness of the pipe without adding corrosion allowance shall be 5.46 mm. Thickness of the pipe after adding 12.5% mill negative tolerance shall be 7.951 mm. Hence, we can select the pipe with the STD Schedule of DN 250 from ASME B36.10 which has thickness 9.27mm.

According to ASME B31.1 mandatory appendix A the minimum specified yield strength for ASME A106 Gr.B material is 35 ksi (241379310.3 Pa).

According to clause 102.3.3 limit of maximum pressure during the hydrotest is to be such that it should not generate stress (longitudinal and circumferenctial) in pipe more then 90% of minimum specified yield stress. So, 90% of minimum specified yield stress for A106 Gr. B material is (35 x 0.90) = 31.5 ksi (217241379.3 Pa).

Following formula is used to calculate the maximum allowable pressure. The same formula is also given in ASME B31.1

P = 2 . S . E . W . (t_m -A) D_o - 2 . y . (t_m -A)

(3)

P = 2 x 217241379.3 x 1 x 1 x (0.00706 - 0.0016) 0.27305 - (2 x 0.4 x (0.00706 - 0.0016))

P = 8842105.263 Pa = 88.42 bar

Where,

S = 90% of the minimum specified yield stress.
P = Maximum allowable pressure.
Rest of the parameters are same as mentioned above.

So, from above example it can be seen that the pipe with design pressure 48 bars can be pressure tested up to 88.42 bars. However usually we take 1.5 times design pressure which gives our test pressure to be 48 x 1.5 = 72 bars which is within limit of 88.42 bars. You will find in most of the cases large difference between value obtained by 1.5 times design pressure and maximum allowable pressure obtained by formula mentioned above. Try few examples your self.