2. Dehydration Plant
Introduction
The dehydration plant is essential to the production of natural gas from the Kandhkot Gas Field.
As the name states this part of the plant is responsible for removing the condensate, including
water and traces of oil, from the natural gas being produced to an allowable level. This is done
to maintain the degree of quality required by the customers which are mainly WAPDA and
SNGPL. A fraction of the gas produced is also sold to SSGCL for use in Kandhkot city.
The main constituents of the Dehydration Plant include:
Finger-Type Sludge Catcher
Scrubbers
Upstream and Downstream filters
Absorber Towers
Glycol Regeneration
Metering Skid
Condenser Handling
Dehydration Unit
Slug Catcher
There are 12 of the finger-type slug catcher installed. The gas from the Gas Gathering Main
lines all go through these and the unsaturated liquid condensate is removed in the process. The
condensate is sent to the condensate tanks while the gas leaves for the Kandhkot Field Gas
Compression Station (KFGCS).
Scrubbers and Filters
The compressed gas coming in from KFGCS comes back to the Dehydration Plant and is then
passed through scrubbers and downstream filters to remove moisture and other impurities
from the gas. The process is very simple, only mechanical separation techniques suffice.
Absorber Tower
In absorber tower most of the saturated condensate is removed from the gas as well. The
natural gas coming from the scrubbers and filters enter from the bottom of the tower and Tri-
ethylene glycol is showered in from the top. Bubble trays are installed in along the tower to
increase the contact time between the gas and the glycol. Condensate is absorbed by the glycol
and leaves from the bottom of the tower towards Glycol Regeneration and the gas leave from
3. the top, ready to be sent to the Metering Skid and eventually the customers. Pakistan
Petroleum Ltd has three functional Absorber Towers currently. Of these three, only two are
used at a time and are sufficient to deliver the requirements and demands of the customer, the
remaining Absorber Tower is in standby in case one of the towers is under maintenance.
Glycol Regeneration
This is a very important part of the process, as the rich or wet glycol is generated to be used
again in the absorber tower. A glycol regenerator is placed near each of the absorber towers. It
is broken into further steps:
1. Glycol Pump
These pumps are used to circulate the glycol in whole of the Regeneration Unit and to
the Absorber Tower. These Kimray double-acting piston pumps are special as they do
not require any electricity or any other power source to function. They are completely
driven by the differential pressure between the incoming rich/wet glycol and the
outgoing lean/dry glycol.
2. Reboiler and Reflux Column
The wet glycol is heated to about 400°F in thee reboiler to remove moisture from it, the
hot vapor is released into the atmosphere via the Still Column. Now the hot dry glycol
passes into the reflux column shell, the cool wet glycol from the Absorber Tower
circulates into the tube-side of the reflux column. Heat exchange takes place and hence
the lean glycol is cooled down for the absorber tower whereas the rich glycol is
preheated before going into the reboiler. Also the unwanted gas in the glycol is
separated in the Glycol Flash Separator and the gas is used as fuel and instrument gas.
4. 3. Glycol Filter
The glycol is passed through a filter too for removal of any trace unwanted substances.
A 40 micron filter is used at the start-up of the unit but usually a 20 micron filter is
employed for normal operation.
Gas Metering Skid
The clean natural gas is collected from the absorber towers and collected into a single line
which is then divided into two major lines to be sent to the customers of Pakistan Petroleum
Limited. Sui Northern Gas Pipelines Ltd (SNGPL) and Water and Power Development Authority
(WAPDA) are the major customers for the gas from the Kandhkot Gas Fields. A small fraction of
the gas is also sold to Sui Southern Gas Company Ltd (SSGCL) to be used in the Kandhkot city.
The objective of this Metering Skid is to ensure the required flowrate and pressure of the gas is
delivered to the customers. Furthermore billing is also done using the metering skid based on
the amount of gas transmitted to the customers.
To check the quality of the gas being sold a Gas Chromatograph is also installed which is used to
determine the components present in the natural gas and their quantity. This is important to
maintain the quality of the gas being delivered.
5. Condenser Handling
A condenser handling facility is also installed in the dehydration plant to manage all the
condensate being separated from the gas. This deals with all of the condensate from the
Kandhkot Plant, including KFGCS. Condensate is first stored in a tank which has three sections.
The condensate is allowed to settle for some time, the water being heavier is removed from the
bottom of the tank and discarded into an injection well; the oil is stored in storage tanks and is
further purified. It is used as fuel or sold as well depending on the need.
Gas Wells
Pakistan Petroleum Ltd has 31 wells currently in its Kandhkot Gas Field out of which only 29 are
functional. The wells are divided into four sections: Sections 3, 4, 5, and 6. There are two Gas
Gathering Mains into which all of the wells have varying lines joining in. These Mains deliver all
the gas from the wells to the plant. Out of these some wells are single and some wells dual; as
in they have two lines going deep into the earth covering two different pockets of natural gas.
Safety is of utmost importance around the gas wells, so they usually have two or more valves to
shut the well down in case of an emergency. A surface safety valve is installed at about 300 feet
and needs a pressure of 60-80 psi to operate, and a Hydraulic valve is installed at 1000-1200
feet into the earth and needs a pressure of 1100-2200 psi to operate. The pressure required to
operate the valves is taken from the gas in the well itself which is ingenious in its own way.
The valves, meters, SCADA system and the other gadgets installed at the wells are powered
from the Solar cells installed. But since the solar units kept getting stolen, the company has also
installed Thermoelectric Power Generation Systems for the wells. This produces power from
the combustion of natural gas and utilizes the principle of thermocouple to do so. The
temperature difference between the two points generates power which is used for powering
the well. The power depends on the flow of the fuel and hence to regulate the power, we
regulate the flow of the gas.
Power Generation
At the dehydration plant there are three generators. Two of which are gas powered and one is
kept as back up and is diesel operated. Even one of the gas generators is enough to power
whole of the Dehydration Plant and the residence for the Officers and the labors. The
generators have their own cooling system, i.e. they are cooled using circulating water.
Power from WAPDA is also available in case of emergencies but as it happens one gas-powered
generator is enough to fulfill the requirements of the plant.
6. Compression Station
Introduction
The Kandhkot Field Gas Compression Station or KFGCS, for short, is a recent addition to the
plant. The main purpose of this set-up is to compress the natural gas from the wells to a higher
pressure so that that the gas wells may have a longer life of service. Another reason why the
gas is required to be compressed is to ensure that the losses in the transmission lines are to be
catered for and the customer receives the gas at their desired pressure. Creating high pressures
for gases ensures that the gas also has a higher dew point which means that the gas will
condense at a higher temperature than it normally would.
The KFGCS houses its own utility section which includes reverse-osmosis processed water,
water for general use, fire water, compressors for pneumatic air and purifiers for Nitrogen gas.
It also has its own power production, maintenance and storage sections.
The Compression Trains
The main section of the KFGCS comprises of three identical compression trains, each capable of
105 Million cubic feet of gas per hour. Out of these only two compression trains are in
operation at once and the remaining stays in back up. The two functional trains are sufficient to
deliver the customer demands.
As was discussed previously in this report, the gas from the wells comes into the plant via the
Gas Gathering Mains. First they go through the finger-type slug catcher which removes the
unwanted unsaturated moisture from the natural gas and from here the gas is brought into the
compression station. The load of the gas is equally divided amongst the functional compression
trains for process. The compression here is done in two stages, cooling takes place between the
two stages.
First the gas comes into the Gas Inlet Scrubbers where the condensate is removed from the gas
coming in from the Slug Catcher. From here the gas is sent down to the Suction Scrubber where
more of the condensate is removed. Then the gas is sent down to the Turbine driven
Compressor for the first stage compression, in which the gas is compressed by 4-impeller rotary
shaft to a pressure of about 32-34 bars. As is common knowledge that increasing the pressure
of a gas results in an increase in the temperature of the gas so is common here too. The
7. temperature is rise to be countered and hence the gas is sent to the first stage coolers. After
being cooled down to an acceptable temperature the gas is then again sent to Inter-stage
Scrubbers to remove any condensate that may have become unsaturated during compression.
From here on the gas comes back to the turbine driven compressor for the second stage
compression which utilizes now three impellers. The gas is now compressed to a pressure of
approximately 47 bars. There is an increase in the temperature again and hence it is now sent
to the second stage fin-fan type coolers. The final temperature leaving coolers is approximated
to around 38°C, which is also the temperature, required by the customers. Finally the gas
moves to the Discharge Scrubber and then is sent back to the Dehydration Plant for further
moisture removal.
The gas lines are constantly sprayed with a Corrosion Inhibitor by NALCO. This is done to
prevent the gas lines from corroding due to moisture present in the gas, this is important as it
protects the pipe lines. Before the gas enters any compression train, A, B, or C, it has to go
through a shut-down valve and a pressure control valve. Now the shut-down valve is for safety
only and is only open or closed. It trips in case of an emergency and cuts off the gas supply to
the train. But the pressure control valve is used to control the flow of gas going into a
compression train. The system recognizes the need to equally divide the load between the
operational compression trains and using the pressure control valves, it regulates the flow of
8. gas going into each of the compression train. These kinds of valves have diaphragms on top and
are regulated by the flow of pneumatic air to them.
The condensate from each of the scrubber is sent via pipeline to the condensate tanks installed
at the Dehydration Plant where they are further processed and the usable oil and the waste
water are separated from each other. The gas from the condensate flash vessel is sent to the
Flare which is always burning and is located a little away from the plant. Even if there is no gas
coming from the flash vessel, a pilot flame is always on. Gas lines are also bled and have
separate lines for the flare joining up together in a single main flare line.
Utilities
The utilities in the KFGCS include Reverse Osmosis Water Treatment and Pumping for use
where clean water is required, pumping of water from the boring pumps, fire water pumping
and circulation, air compressors for use in PSVs, PCVs and LVs, and air purification system to be
used purging of gas lines.
1. RO Water Treatment
Water obtained from the ground water sources have to be treated before it can be
used. Two Reverse Osmosis units are set up out of which one is functional at a time and
the other is kept as backup. The water treated and pumped from here is used for
sanitation in washroom, cleaning, etc. all over the Kandhkot Field, including the
residence.
2. Boring Water Pumping
This is the water used for small utilities all over the plant where the quality of the water
can be compromised. The water obtained from the ground sources is simply pumped to
the areas where it is needed. Here also two units are in place, one as backup and one for
use.
3. Fire Water System
There are two sets of pumps installed for the fire water system. Two are regular pumps
which are responsible for maintaining the minimum pressure of 10 bars all over the fire
water points and hoses. In case the fire water is in usage and the pressure falls below 10
bars and the regular pumps are unable to handle it, two large fire safety pumps are also
in place. These are set to automatic on and off. Every time when the pressure drops
below 10 bars these turn on automatically and provide the needed pressure in the fire
water lines. Fire water system has its own dedicated water storage tank so that in case
of an emergency there is ample supply of fire water. The lines for fire water all over the
plant are distinguished by their red color.
9. 4. Air Compressor
There are two air compressor units installed and their purpose is solely to compress air
for use in pressure control valves (PCVs), lockout valves (LVs) and pressure safety valves
(PSVs). These valves have proportional-integral-differential (PID) controls and need
compressed air to function. The valves are either air-to-open, meaning that the valve
will open when there is air supply to it, or air-to-close, meaning that the valve will close
with the pressure of air it receives. Compressed air is absolutely essential to the plant;
otherwise the whole of the plant will be forced to shut down. In essence the air
compressors play the role of the heart for the plant.
5. Air Purifiers
These units are responsible for taking in the air from the environment and purifying it so
that only Nitrogen gas remains as a product. Nitrogen is then used for the hydrocarbon
purging, which means that if a line is to be cleaned Nitrogen is sprayed into to clear it of
any hydrocarbons so that if some mechanical functions are performed then there are no
hazards or risks of damage and injury. It is also used in sealing of the components. As
Nitrogen gas is unreactive and is available in abundance so it is the ideal gas to be used
for these situations.
Air compressor takes in the air from the environment and compresses it to a pre-
defined pressure, the air is then sent to the air buffer which basically removes any
moisture from the air. The absorbers remove Oxygen from the air so that the air
becomes unreactive. Pressure swing absorbers are used, which means that they
function alternatively, the first absorber will shut down and the second will start
automatically after a 5-6 min interval and this goes on. The buffer vessel is just for
keeping the Nitrogen gas until the booster compressor sends it to the desired location.
10. Power Generation
Like the Dehydration Plant, the Kandhkot Field Gas Compression Station also produces its own
power for the plant. Here the company has three gas generators, each capable of producing
1750 kW of power, one diesel generator, which is mainly used as backup.
All three gas generators are identical and at any given time only two of them are operational.
The third is kept as a backup, incase of any maintenance operation that has to be performed on
any generator. Two of these produce enough power to cater for the whole of the KFGCS. The
generators have 18 V-type cylinders, divided into A and B sections. Engine rotates at 1500 RPM
which drives the generator and hence produces electricity.
Generator obviously heats up so to keep the temperature in check three cooling towers are
installed. Two of these are enough to bring the temperature of the water to the desired value
and one is kept in back up. Lube oil circulates within the generator which cools the generator,
water from the cooling tower cools the hot lube oil and this process goes on. Obviously the
cooling water and the lube oil do not come into direct, physical contact, only heat exchange
occurs between the two through pipelines.
The cooling tower is Induced Draft type, as it has fans installed on the top. The temperature of
the water being cooled in the tower cannot go lower than the ambient temperature. Some
coolants are added to the water to enhance the process of cooling the generators and the pH of
the water has to be maintained because otherwise it will cause corrosion in the lines.
Waste Water Disposal
Condensate is collected from all the scrubbers in the KFGCS, from the filtration units in the
Dehydration Plant and the sent to the condensate settling tanks. Here the water and the oil are
separated as they are allowed to settle. Water settles to the bottom and oil stays on top. This
procedure is time taking and the condensate must be left for days on end for the water and oil
to be effectively separated.
The water from her is pumped to a ditch near the KFGCS which is lined with rubber sheets so
that that water does not seep into the ground and contaminate the soil. The water from the
ditch is then pumped to an injection well outside the boundaries of the Kandhkot Field. This is
done using Rota-pumps so that the water is injected deep into the injection well and does not
intoxicate the earth above.
Waste water disposal is quite necessary as it becomes the company’s responsibility to deal with
all such chemicals or by-products produced effectively and without adverse consequences to
the local environment which might in turn effect the local population.
