Pigging process in Delayed Coker Heater Units

Process of removal of coke deposited inside the tubes of the heater to facilitate smooth and continuous operation called as pigging. Water pressure is used to push Styrofoam Pigs with studs and grit on the exterior through the tubes to remove the coke. Pigging is needed when skin Temp.in Radiation zone reaches 670°C or Inlet Pressure reaches 50 Kg/cm2g.

In order to shut down the heater for pigging;

Ramping down the pass flow to 50m3/hr.

Increase Velocity Steam in all passes.

Once flow reaches 50m3/hr firing to be cut off.

When COT reaches 400 c isolate the manifold and start to flush.

Increase the velocity steam up-to 1.5tph per pass.

Vapor diversion from fract to BD after 30 min.

Further increase steam up-to 4 tph and check for any HC traces.

Switch over to next drum keeping the off line drum vent open.

Isolate the steam and depressurize the coil

Allow it to cool and issue the permit of elbow dropping once the temperature reached 100c.

The next step is to fix the pigging elbow;

Once if the elbow dropping is completed then pig launchers is to be fixed.

4 – no’s at the convection inlet & 4 no’s at the radiation outlet.

Pig launchers are the temporary bores used to push styrofoam pig with the aid of water at higher pressure.

The next step is to perform the flow test;

These flow tests are conducted to know what is the reduction of pressure drop achieved before and after pigging for maintaining the same flow.

During the flow test the inlet pressure, flow rate and machine RPM will be recorded.

Start up heater after pigging;

External surface cleaning : The coil outer surface will be cleaned simply by scraping.

Floor cleaning by vacuum appliances.

Box up :                                                                       

Man way box up permit has to be issued after thorough checking inside the heater for presence of any foreign material.

The next process is de-blinding;

Once if the man ways are boxed up with the minimum bolts we can go for deblinding as per the list.

Most priority :

                  1) Box purge steam.

                  2) Pilot gas

                  3) Fuel gas.

Box purging:

After removing condensate from the purge steam manifold steam is to be admitted to the radiation box .

This steam will heat the air for producing the draft .

Start the ID fan and simultaneously isolate the box purge steam.

Open N2 and PG isolation valves.

Start the PG burners through BMS sequence.

BMS Start up sequence;

Press ‘System Start’

Pressurisation -  Leak test

Depressurisation

Purging – (5 times of furnace volume in 5 minutes)

Pilot Start’ - press pilot ignition PB

‘Fuel Gas Start” - open manual valve

Pressure test;

Pressure test is conducted at 40 Kg/cm2 using HP steam to check any leakages at the elbows.

Steam is continuously admitted to the heater coils till the pressure reaches 40 kg. While the transfer line PSV common u/s isolation valve should kept close.

After successful completion of coil pressure test , PSV loop has to be tested at 10 Kg/cm2.

Establish oil circulation through the coils to fractionator.

Liquid Heating of coke drum to rise the CD temp upto 200-220 deg.C

Rise COT upto 400 deg.C

FEED-CUT IN

Calculation on heater down time:

Coil steaming = 06 hrs

Elbow reversal & launcher fixing = 10 hrs

Pigging = 20 hrs

Swing elbows box up = 06 hrs

Pressure test/oil circulation & Liq heating = 10 hrs

TOTAL  = 52 HRS