Pneumatic Testing - Safe Distances from Stored Energy Calculations

Pneumatic testing is often used to test piping systems when the presence of moisture is undesirable or removal of residual water cannot be conveniently achieved. Due to incompressibe nature of hydrotest fluid, the energy stored in a piping system under hydrostatic pressure is far less in comparison to a piping system which is subjected to pneumatic pressure. 

The stored energy of a compressed gas is significantly higher and hence rupture of a piping system during a pneumatic test can release large amounts of stored potential energy into kinetic energy which results in rapid expansion (explosion) and makes it very unsafe. The time gap between a leakage and failure is very small making it impossible to take any mitigation measures. 

Blast overpressure is the increase in air pressure caused due to a shock wave caused by an explosion. Overpressure can cause injury to personnel and damage to surrounding plant facilities. Some of the damaging consequences of blast overpressure are tabulated below:

Source: Glenn Research Center-Glenn Safety Manual GLP-QS-8715.1.7 - Pressure Systems Safety

Prior to commencing the pneumatic test, the exclusion zones should be clearly identified. Exclusion zones can be defined as boundaries beyond which the effects of overpressure or fragment throw (whichever is applicable) will not cause damaging effects to personnel or plant facilities during pressure testing.

Stored Energy Calculations and Blast Wave Safe Distances

ASME PCC-2 Mandatory Appendix 501-II and III provides equations for stored energy calculations and minimum safe distances between all personnel and the equipment or piping system being tested.

• The first step involves calculation of stored potential energy which is a function of system test pressure and the volume being tested using equation II-2.
• The second step involves converting the stored energy into equivalent kilograms of TNT using equation II-3.
• The third step involves determining the safe distances which will be greater of the values calculated based on section 501-III-1(a) and 501-III-1(b).

Section 501-III-1(b) involves converting the TNT values to safe distances using default value of R_scaled = 20m/kg^(1/3) or greater. If the value of stored energy exceeds 271,000,000 J, section 501-III-1(a) does not apply and the safe distances shall be calculated using equation (III-1) referenced in section 501-III-1(b). The example in the following section provides a step by step solution for performing the stored energy calculations.

The 2018 version of ASME PCC-2 states "When calculating the stored energy of piping system, a maximum volume based on a length of 8 pipe diameters may be considered for single failure analyzed." This approach is perhaps based on the premise that the weld-length failure which contributes to the blast wave may not exceed 8 pipe diameters.  Mandatory Appendix 501-IV does highlight that the use of 8 pipe diameters may not be sufficient in certain cases. Many organizations would tend to use the conservative approach and calculate the complete volume of pressurised section. For example Alberta Boiler Safety Association (ABSA) as on the date of publication of this article have chosen to disregard this section for stored energy volume considerations.

Example of Stored Energy Calculation

Problem: A piping flare system of 24" NPS wall thickness, STD (9.53mm) wall thickness and length 600m is subject to a pneumatic test pressure of 10 barg. Calculate the minimum distance between all personnel and piping being tested. 

Solution: The first step involves calculation of stored potential energy using the following equation:

Step-1 Calculation of Stored Energy

Stored Energy (E) = 2.5 * P_t * V \(\left[1-\left(\frac{P_a}{P_t}\right)^.286\right]\) ..... as per equation II-2 from ASME PCC-2 Appendix 501-II.

where 

P_a = absolute atmospheric pressure = 101,000 Pa

P_t = absolute test pressure

V = total volume under test pressure

Pipe ID = 610 - 2*9.53 = 590.94mm = 0.59094m

Pipe Volume (V) = π*(0.59094)²*600/4 = 164.56m³

Test Pressure = 10 barg = 10*100 kPa  = 1000kPa = 1,000,000 Pa

Absolute Test pressure (P_t) = 1,000,000 Pa + 101,000 Pa (absolute atmospheric pressure) = 1101,000 Pa

Substituting in above equation II-2, the values of P_a, P_t and V gives us

Step-2 Convert the stored energy into equivalent kilograms of TNT

The next step involves converting the stored energy into kilograms of TNT

TNT = \(\frac{Stored Energy}{4,266,920}\) (kg) ..... as per equation II-3 from ASME PCC-2 Appendix 501-II

Step-3 Determining the safe distances based on values of stored energy

As per ASME PCC-2 Mandatory Appendix 501-III the following minimum safe distances shall be maintained depending on value of stored energy and shall be greater of the values calculated based on section 501-III-1(a) and 501-III-1(b):

As per section 501-III-1(a):

If E <= 135,500,000 J, the safe distance to be maintained is 30m.

If E > 135,500,000 J and less than equal to 271,000,000 J, the safe distance to be maintained is 60m.

If E > 271,000,000 J, then section 501-6.2 (f) of ASME PCC-2 prescribes the following:

• Division of the piping system into smaller volumes such that E becomes less than 271,000,000 J.
• Provision of barricade that can withstand the blast of stored energy.
• Calculating the safe distance (R) in accordance with equation below.

Since the value of stored energy is 224,057,795 J the safe distance to be maintained as per this section is 60m.

As per section 501-III-1(b):

Safe Distance (R) = R_s(2TNT)^1/3..... as per equation III-1 from ASME PCC-2 Appendix 501-III.

where R_s = scaled consequence factor whose minimum value shall be 20m/kg^(1/3).

Substituting the values of TNT = 52.51kg and R_s =  20m/kg^(1/3), the safe distance to be maintained as per this section is R = 94.36m.

When the above spacing requirements cannot be practically achieved, ASME PCC-2-2018 permits design, fabrication and installation of a barricade which is capable of withstanding the blast of stored energy within the system.

Stored Energy Calculations and Safe Distances for Fragment Throw

Failure of a piping system can occur due to either ductile or brittle fracture. A brittle fracture has the potential to generate a large number of fragments. The 2018 edition of ASME PCC-2 includes section on minimum safe distances for fragment throw considerations. This section should be used when fragments of piping are at risk of being created and impacting personnel. The safe distances to be maintained based on fragment throw considerations are larger when compared to distances based on blast wave considerations. In the example above, for a TNT of 52.51 kg the minimum safe distance to be maintained is approximately 94m. From Table 501-III-2-1 of ASME PCC-2, the safe distances for TNT of 50-65 kg based on fragment throw considerations is 130m.

By maximizing the use of non-destructive examination to ensure integrity of the welded joints and establishing safe testing procedures and exclusion zones, the probability and consequence of failures can be reduced thereby minimizing the risks associated with pneumatic testing.