Residual ammonia released from compressor shaft seal triggers 100 ppm ammonia alarm

Reference number
II-690530-2018 BPVII-1120802-2018
Cover plate

Incident overview 

Residual ammonia released from compressor shaft seal triggered 100ppm ammonia alarm and premises were evacuated till it was confirmed leak in the machine was insignificant and everyone was accounted for and safe.

Regulated industry sector

Refrigeration system

Location

Vancouver

Ammonia leak detection and alarm system channel 2 sensor located above compressor No 1

Investigation conclusions

Site, system and components

The refrigeration plant is in a public assembly site-ice rink. Plant has one critically charged indirect closed loop vented single stage vapour compression refrigeration system (critically charged means chiller/evaporator can hold whole ammonia charge without letting liquid ammonia into compressor suction) with 650 lbs of anhydrous ammonia in primary side and about 1000 gallons of brine solution on secondary side. In primary side, two single stage reciprocating compressors compress low pressure ammonia vapour to high pressure vapour which is condensed to high pressure liquid at condenser. High pressure liquid is fed at the bottom of shell side of the Chiller through Armstrong inverted bucket expansion valve (ammonia pressure drops while passing through orifice of expansion valve) and liquid ammonia submerges all tubes in the chiller. Brine solution is circulated through tubes in the chiller (ammonia is on outside surface of tube and brine passes through tubes-there is no physical contact between ammonia and brine) and coils underneath floor of the arena by brine circulating pump. Receiving heat from brine circulating through tubes ammonia turns into vapour and vapour pressure of ammonia in chiller is maintained low by two compressors. Resulting colder brine circulating through coils produce ice surface in arena.


Shaft Seal of Compressor: Shaft seal consists of stationary and rotating parts and prevent ammonia from leaking out through gap around shaft where shaft extends out of compressor crankcase. Shaft Seal collar with o’ring in its inner surface groove and directly mounted over the shaft with locking ball and lock nuts, rotates with shaft and against stationary shaft seal ring with o’ring in its outer surface groove and held in the end cover of the compressor housing against a set of helical springs. A thin oil film formed between two precisely machined sealing/mating faces lubricates and maintains effective sealing and springs maintains steady contact pressure between these sealing faces. Circulating oil (Compressor lubricating oil pump constantly circulates lubricating oil to bearings and shaft seal when compressor is running) provides lubrication (facilitates ease of axial movement of sealing faces as required to maintain effective seal), cooling and forms thin oil film between two sealing faces which eliminates frictional wear of sealing faces.

Failure scenario(s)

Compressor was in idle condition for some time. Lack of lubrication and diminished oil film between stationary and rotating sealing faces of shaft seal (shaft seal chamber is continuously supplied with lubricating oil by compressor oil pump while running) resulted in ineffective sealing between sealing faces allowing ammonia from compressor crankcase to leak into room.

Facts and evidence

There were two witness accounts.

  • First was certified operator (responsible for operation of the plant for the duration of morning shift). During his regular round at about 09:15am he entered machine room with portable/handheld ammonia detector after confirming 0ppm ammonia reading at ammonia detector monitor by the vestibule. He smelt ammonia despite his monitor showing zero ppm. So he went out of machine room to conduct bump test for his detector. He went back to machine room after successful bump test and he noticed his detector red 54 ppm when he had stood very close to No.1 compressor shaft seal. As soon as he had moved away from No.1 compressor his detector read 0ppm. He also noticed that ventilation fan was running at high speed. He texted chief engineer of the plant. Chief engineer through another off duty operator instructed him to call refrigeration mechanic from licensed refrigeration company to come to plant. Upon arrival on site at 11:30am refrigeration mechanic isolated and purged compressor and both of them left machine room. Strobe light and alarm for 100ppm ammonia leak went off at 12:30pm. Not finding the refrigeration mechanic and consulting with the operator plant supervisor ordered evacuation and called Fire Department at 12:40pm.
     
  • Second account was certified refrigeration mechanic from licensed refrigeration company. Upon arrival on site at about 11:30am he read zero ppm for both channels/sensors on machine room ammonia leak detector monitor and went to machine room. Hearing from what operator experienced and smelling ammonia close to No.1 compressor shaft seal, he isolated and purged compressor to ensure no ammonia vapour pressure left in the compressor. He left machine room for lunch break along with operator who headed out to attend different plant job. Having been informed of 100ppm ammonia leak in the plant by one of his company colleagues over phone10 minutes into his break, he went back and met operator exiting machine room. He noticed plant monitor read 45ppm for channel 2. He donned full face mask entered machine room. He noticed increase in ammonia reading after removing water bucket into which he purged ammonia (most likely agitation of water while walking caused more ammonia release from water). Machine room monitor read zero for both channels/sensors while he was exiting machine room to meet others. Meanwhile Fire Department was called and upon arrival fire fighters raised fire alarm and made checks to ensure everyone in the building was accounted for. After final visit to machine room with the refrigeration mechanic and one of the Vancouver Board of Parks and Recreation employees Fire Department reset fire alarm, declared plant is safe to return and left site at about 2:30pm.

Log history of ammonia detector validates:

- Ammonia leak was detected by sensor 2 (channel 2) over compressor 1 at low level few times on May 17, 2017 indicating leak emanating from compressor 1(not noticed by operator as low level alarm auto resets once level goes down);

- High level alarm was detected by sensor 1 (channel 1) close to vestibule.

Service reports from licensed contractor stated that refrigeration mechanic, who attended the site for follow up at night on May 18, 2018, had found suction and discharge valve gland packing leaking and crankcase oil heater on and refrigeration mechanic, who inspected shaft seal on June 12, 2018, had found lock nuts for shaft seal collar loose.

  

II-690530-2018,:BPVII-1120802-2018 3

Causes and contributing factors

It is highly probable that prolong idling of compressor caused loss of lubrication and thin oil film between stationary and rotating sealing faces to diminish. Other probable contributing factors are: O’ring of shaft seal collar might have roughened or fretted (as effect of loose lock nuts) losing ability to seal effectively. Hasty closing of compressor shut off valves without following proper procedure or deterioration of gland packing material over time resulted in gland packing leakage. With crankcase oil heater on, ammonia in oil might have evaporated and continued leaking through shaft seal as crankcase pressure had built up by evaporated ammonia. Evaporation of ammonia from water bucket into which ammonia from compressor was purged after isolation also contributed to 100ppm ammonia alarm 

Impact

  • Injury
    • Qty injuries: None
    • Injury description: None
    • Injury rating: N/A
  • Damage
    • Damage description: Loss of effective sealing function of compressor shaft seal.
    • Damage rating: Moderate

Incident rating

Moderate