Ammonia Release | Pinhole Leak

Summary

In September 2016, the development of a small hole in the wall of an ammonia suction pipe resulted in a release of 15 lbs of ammonia. Neglected insulation breaches allowed moisture to accumulate on carbon steel piping. This resulted in corrosion and eventually, a pinhole developed in the wall of the pipe. 

Release Calculation

The release calculation was performed with the following assumptions:

• Leak Duration: 40 minutes
• Shape of hole was assumed to be an ellipse
  • Minor radius = 0.0375 in
  • Major radius = 0.0755 in
• Pressure: 33.5 PSIG (corresponding to 20ºF) 
• Molecular Weight of Ammonia: 17.03 g/mol
• Pipe insulation did not mitigate the leak 

A challenging aspect of the release calculation was the shape of the hole. As stated above, the hole was assumed to be in the shape of an ellipse, which is more complicated to calculate compared to a perfect circle. The amount of vapor released from a circular hole was calculated as follows:

Vapor Release

Dow Chemical Exposure Index Guide, 1st Edition – Equation 1B (Page 11)

AQ = 3.751D2PaMWT+459

where,

AQ = Airborne Quantity (lb/min)

D = Diameter of hole (in)

Pg = Gauge pressure (psig)

Pa = Absolute pressure = Pg + 14.7

MW = Molecular weight of ammonia (17.03 g/mol)

T = Temperature (ºF) 

Adjusting the equation to allow for an ellipse-shaped hole, rather than a circle results in:

 AQ = 4.776APaMWT+459

where,

AQ = Airborne Quantity (lb/min)

A = Area of hole (in2)

Pg = Gauge pressure (psig)

Pa = Absolute pressure = Pg + 14.7

MW = Molecular weight of ammonia (17.03 g/mol)

T = Temperature (ºF)

 Area of an Ellipse

A = a b

where,

A = Area (in2)

a = minor radius (in)

b = major radius (in)

 A = (0.075 in2) (0.151 in2) = 0.0089 in2  

Airborne Quantity

 AQ = 4.776 (0.0089 in2) (48.2 psia)17.03gmol20℉ + 459 = 0.386 lb/min

Total Ammonia Released

Ammonia (lb) = 0.386lbmin 40 min = 15.44 lb 15 lb

Significance

IIAR 6-2019 §11.1(b) states “Visually inspect for damage or moisture incursion in insulation (i.e., dampness, condensation, frost, ice buildup)”. Damaged or saturated insulation allows moisture to corrode the piping underneath. This pinhole leak appeared to be directly caused by corrosion under insulation” (CUI) and possibly could have been prevented by quickly addressing saturated insulation. Facilities should ensure they have implemented a comprehensive pipe inspection and maintenance program to ensure CUI is caught, and addressed before it’s too late.