Alberta Special Oilwell Boiler Practice Test

The lift differential is the extra pressure above the valve’s set pressure that’s needed to move the safety relief valve off its seat to full opening and allow adequate relief flow. In Alberta practice, the maximum lift differential is defined as the greater of three percent of the set pressure or 20 psi. This rule ensures enough lift to provide sufficient discharge across a range of set pressures, while also preventing overly small lift at low set pressures that could cause nuisance operation. For example, if the set pressure is 200 psi, three percent is 6 psi, but 20 psi is the governing value, so full lift occurs at about 220 psi. If the set pressure is 1000 psi, three percent is 30 psi, which is greater than 20 psi, so full lift occurs at about 1030 psi. This standard keeps the valve reliable and capable of relieving at the intended rate across different boilers and pressures.