In the concentrator section of phosphoric acid plant, weak phosphoric acid is concentrated by evaporation in graphite made evaporator. This is normally operated under vacuum & ~88°C temperature due to highly corrosive nature of phosphoric acid (PA). Over a period of time, the tubes of this exchanger require cleaning for removal of deposits & fouling material. This is a normal practice in both plants i.e. PAP-I & PAP-II.
The weak acid is filled in the tube side of this exchanger at atmospheric condition & after closing the system &amp;amp;amp; pulling the vacuum; steam is injected for raising the temperature. This steam is fed into the shell side of this evaporator @ 3.5 to 4 kg/cm2g. The condensate formed is collected in a small vessel of ~2 m3. This condensate is pumped to the main condensate storage tank CST-1 in main CPP plant.
The vent from this small condensate tank of PAP plant goes to barometric condenser for sealing, which was isolated later on. From the top of the condensate tank, one more line of ½” is provided which acts as an overflow drain. The discharge system is having one conductivity meter, which is used to monitor the quality of condensate being sent to CST-1. In case, conductivity of discharge condensate increases, the pump discharge is manually diverted to the local drain in cooling tower system to avoid any contamination in CPP.
- The said evaporator was under hydro jet cleaning on 26th Oct’2004 in PAP-II.
- AAfterwards, it was taken under filling with weak PA at @ 2004 hrs.
- The PA acid filling rate was 75 m3/hr @2034 hrs.
- @ 2034 hrs, the level in condensate tank in PAP-II was 3.9%.
- The conductivity meter was showing a normal reading of 16 -cm/s.
- @ 2040 hrs, the condensate tank high-level alarm appeared in control room.
- Till the time, steam was not going to the evaporator.
- @ 2150 hrs condensate pump was started to transfer the liquid from condensate tank to CPP, but it tripped.
- The conductivity meter was showing a reading of 1.5 -cm/s.
- @ 2150 hrs it was found that inlet drain of this pump was in choked condition & pump was tripping on overload, which Electrical Deptt did inform.
- @ 2250 hrs the drain of pump was dechoked manually & it was found by the operator that PA acid was coming out form this drain.
- Therefore, acid filling in the evaporator was stopped @ 2250 hrs.
- @ 2300 hrs the draining of evaporator was started.
- Meanwhile it was found in the smelter WHB area & in AFBC boiler area that the gaskets on joints were leaking out after cracks & PA acid with water was coming out from them.
Comments after Analysis
After discussion with all concerned staff of PAP plant, Technical improvement Cell (TIC), CPP, Water treatment section & other executives, CTC came to following conclusive points.
- The first apparent incidence was the condensate tank high-level alarm @2040 hrs. When there was no steam inlet to the system, this condensate tank high-level alarm should not appear. It indicates tube side leakage in the evaporator.
- The indication & behavior of conductivity meter was erratic. If actual condensate conductivity goes high beyond its full range value of 16 it resets itself & shows a reading of say 19 as 3 -cm/s. This kind of behavior of any instrument is very uncommon in the industries. In this case, it should report stack or data overflow.
- Because of resetting of the conductivity meter reading the operator is not aware of current range of actual value whether it is over critical limit or under critical limit at any point of time. So this instrument becomes irrelevant for the operational decision. In fact, it can be dangerously misleading.
- Because of new value of 1.5 -cm/s, operator cannot immediately judge/identify the problem of high conductivity or any leakage in the system.
- The liquid filling rate of 75 m3/hr / normal steam consumption rate of 35 to 40 TPH suggest that the capacity of 2 m3 of the existing condensate tank in PAP plant is not designed to hold acidic water .
- It should have sizeable top / overflow line, which can take care of any leakage in the system considering its corrosive nature. The size of drain line is not sufficient to take up even 10 m3/hr influx of liquid. So the leakage influx could not be drained out.
- There was no second measure for safety in this critical system. Considering its highly corrosive nature it should have at least two simultaneous systems to avoid any criticality due to system & operational failures.
- Material of construction (MOC) of the existing evaporator in PAP-II is from M/s. Graphite India whereas; PAP-I evaporator is from M/s. Le-Carbon. Based on our past experience, we suggest that the quality of Le-Carbon make graphite is superior to Graphite India Make. Therefore, Unit should replace the MOC of existing evaporator with Le-Carbon make in the next available opportunity.
- Unit should install 2 highly reliable conductivity meters (it should have display range up to 100 -cm/s), also if conductivity exceeds this limit, indication should stay at maximum value.
- One of this conductivity meters should be installed in incoming line of condensate, which is transferring the steam condensate from evaporator to the new hold up tank, with a provision of three-way valve. In case, conductivity goes beyond normal value (say 16), it will divert the condensate feed from holdup tank to drain pit.
- The holdup tank should be provided with a LIC & level control valve in the discharge of pump. The pump motor should also have startup interlock with high conductivity.
- In the transfer line from holdup tank to CST in CPP, there should be another meter with three-way valve. In case, conductivity goes beyond normal value (say 16), it will divert the condensate from pump discharge to drain pit.
- Both of the proposed three-way valves will operate in one direction either in draining mode or in-line mode based on the higher reading out of two conductivity meters.
- Overflow drain line should be 2” in size.
- It is also recommended to train the staff on HAZOP aspects after detailed HAZOP study of PAP plants.