Offshore & Marine Chiller Service

Offshore oil rig showing chiller service being carried out in marine environment

Offshore & Marine Chiller Service

North Sea Offshore & Marine Chiller Service

On a previous visit for offshore & marine chiller service on an oil rig, our customer had shown us to a large air cooled chiller. His complaint was that it was tripping out all the time when there was nothing wrong it. Complicated ‘in house’ controls, where a string of PCBs, plugged into each other were found in the panel. We recommended a controls retrofit and received the order to carry out work…

Finicky Controls

All of the finicky control PCBs were removed from the panel and placed in the electrical recycling skip.

Offshore & Marine Chiller Service to Control Wiring 

The control wiring was removed and grouped together according to colour. This was because the same wiring was going to be re used to re wire chiller.

Inverters

Two Danfoss inverters were installed into the panel. These were to provide a VSD (variable speed drive) for two large screw compressors.

Electronic Expansion Valves

Danfoss EEVs were easily brazed into place as both systems could be pumped down and valved off. This part of the fridge system was then nitrogen pressure tested then vacuumed according to standard industry guidelines.

Fan Speed Controllers

Both systems had a Danfoss fan speed controller fitted to the chiller frame to control the condenser fans. Then, the control transducers were wired in to the FSCs. This made them stand alone and capable of independently controlling the head pressure of each system.

Danfoss Chiller Controller

The controller in the photo is capable of running the inverter drives for the compressors and the electronic expansion valves. It was wired in, one wire at a time: first the Inputs, then the Outputs and finally the power supply.

Offshore & Marine Chiller Commissioning Service

We always carry out commissioning in the morning as this is the best time of the day to solve problems. It was time for a cup of coffee and a deep breath. Our engineer had only re wired one panel previously and he was also a long way from the office.

Switch On

When he switched the power supply on, there were no immediate problems, so he waited for the for the timer to count down. System 1 started and after a while System 2 started too. There were a few teething problems with the settings, but then the chiller settled down. No more spurious trip outs.

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Controller used in offshore chiller service
Cheap and very reliable

Offshore & Marine Chiller Service in Liverpool

A bulker had got into difficulties in the Irish Sea and been towed to Liverpool for service. It was discovered that a rope had got wrapped around the propeller. Aside from this, one of two chillers which are used for the cooling of the living quarters had tripped out. The Chief Engineer had therefore decided to kill two birds with one stone. Most of the crew were from around Tanger Med in Morocco and so spoke only a little English. They also have limits as to how far they will go with a fridge system. Not to worry though: all we need is to see the plant and we can take it from there...

Seawater Cooled

The plant had a sea water cooled condenser which was fed via a pump set in an adjoining plant room.

Offshore & Marine Chiller Service to Controller

The controller alarm message stated that there was a low pressure condition in the evaporator. On interrogation of the menu, however, the pressure of the evaporator was found to be 7 bar which was the same as in the condenser.

Reset

Our engineer therefore decided to follow through the reset procedure. The alarm cleared and the machine started up. Then, it pumped down to 1 bar and tripped out on the same fault.

Liquid Line Solenoid Valve

While this was happening, our engineer was quick to notice that the liquid line solenoid valve had not activated. He decided to manually jack it open with his test equipment and the refrigerant rushed through okay.

Liquid Line Solenoid Coil

Having decided that the valve was okay, he then investigated the coil. The connection box and the wiring was found to be in poor condition.

Isolate

He then isolated the power supply and checked that it was dead.

Rewire

The wiring was cut back and he re made the connections onto the solenoid coil without using the connection box. Then, he insulated the area with a special compound.

Test Operation

Now to see if he had got the diagnosis correct. He powered up the chiller and carried out a run test- all readings were found to be nominal and the machine went back into service.

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Planning for Offshore & Marine Chiller Service

The main part of the planning for an offshore visit is done on the previous job. A detailed list of the required parts, materials, refrigerant, tools and equipment is composed on our Offshore Manifest Sheet. The questions and headings have been designed to prompt an engineer into thinking about all aspects of the job in detail.

Preparation for Offshore & Marine Chiller Service 

Having emailed the above mentioned sheet to Head Office, we cross reference the information with the engineer while he is still offshore. All this preparation is to ensure that the upcoming job goes as planned. There would be nothing worse than to have to postpone the job due to a lack of preparation.

Steel Chests

We have several steel chests that we use for shipping to oil rigs. After the parts and materials arrive at Head Office, they are checked off, one at a time into the shipping chests. Then, the necessary tools and equipment is checked off too.

Offshore Stores

Our customers also have their own equipment in their stores. They have things like fittings and fuses too. This is a good backup for when something is unforeseen, or if something breaks.

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Showing how do chillers work - brass expansion valves inside machine

How do Chillers Work? Expansion Valves

In this article we are going to explore: how do chillers work- expansion valves. They are also referred to as a metering device, as they control the flow of refrigerant into the evaporator. Read below for an insight into the 4 main types that are used on chillers…

How do Chillers Work - Electronic Expansion Valves

This is a refrigerant control which uses a mechanical valve operated by a step motor. A step motor opens the valve according to the number of steps in between the closed and the open positions. This step motor is controlled by a program in the chiller controller…

On Board Comparator

Every engineer has a comparator app on his phone and a set of comparator sliders in his tool case. The controller has sensors fitted to the evaporator and an in built comparator for the refrigerant which runs through the system. The comparator is a scale which compares the pressure of the refrigerant to the corresponding saturation point. Saturation point is another way of describing the boiling point of the liquid refrigerant.

Superheat

The above algorithm is needed to decide the position of the step motor. A pre programmed amount of superheat (the temperature above saturation) is programmed into the controller. This prevents compressor failure due to liquid refrigerant slug back.

Efficiency

Because the above is controlled by a step motor and a microprocessor, the superheat is closely controlled to as low as 1°C. A low superheat value is one of the factors which contributes to a machine running more efficiently.

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How do Chillers Work - Solenoid Expansion Valves

This is a very simple expansion valve. It is just a solenoid valve fitted into the liquid line which is operated by a solenoid coil. The control of this valve is very similar to the above. The difference is that the program works out how long to keep the valve open and how long to keep it shut.

How do high side float expansion valves work when welded into steel ammonia chillers pipework
Refrigerant control on flooded system

How do Chillers Work - High Side Float Expansion Valves

The photo shows this type of valve as used on an ammonia chiller. This kind of chiller is a flooded system where all of the refrigerant is stored in its liquid phase on the low side of the system. As load increases: more refrigerant boils off in the evaporator- it is sucked into the compressor- discharged into the condenser and then arrives as a liquid at the float valve. When the chamber in this valve fills, it lifts a float inside which is made of steel. The float operates a switch which sends volts to the liquid pipe solenoid valve. When this valve opens, it lets the liquid in the float chamber to pass into the evaporator. Having done this, the float drops back down and the valve closes again. The closing of the valve maintains the pressure difference between the high side and the low side of the system.

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How do Chillers Work - Thermostatic Expansion Valves

There are 4 forces acting on a TEV: the power element v spring and liquid line pressure v suction pressure. The expansion valve has been designed to balance these forces.

Power Element

A bulb is fitted to the suction pipe near to the compressor. This bulb is charged with the same refrigerant that is running in the chiller system. It is connected to the power element using a capillary tube (photo at the top of the page) When the temperature rises in the evaporator, liquid refrigerant boils off in the bulb, this forces a diaphragm to open in the power element.

Spring

A needle located on the outside of this diaphragm pushes the spring down. This opens the expansion valve and lets more refrigerant through.

Liquid Line Pressure

The liquid line is the inlet to the expansion valve and carries the refrigerant arriving from the condenser.

Suction Pressure

The pressure drop on the far side of the expansion valve is maintained by the sucking action of the compressor.

Mass Flow Rate

The above allows for a high mass flow rate when the evaporator warms and a lower mass flow rate when the evaporator cools.

Liquid Line Solenoid Valve

To prevent refrigerant passing into the evaporator during off cycles, a liquid line solenoid valve is often used. This type of system usually pumps the evaporator down before the compressor stops. Shortly before the compressor starts back up again, the liquid line solenoid valve opens, the pressure increases in the evaporator and then the compressor starts.

Equalizing Line

This is usually a copper pipe which connects the expansion valve to the far side of the evaporator. It compensates for the pressure drop across the evaporator. It also detects the pressure of the suction pipe at the same location of the sensing bulb. This allows the TEV to accurately adjust the superheat.

Hot Gas Bypass

Some chillers, like laser chillers, require a very close control of setpoint. Sometimes the deadband can be as narrow as 0.2°C. If the compressor were to stop and start all the time to achieve this- it would fail very quickly. Therefore, a hot gas bypass can be used to allow the compressor to carry on running when the chiller achieves setpoint. A solenoid valve opens which allows the discharge hot gas to bypass the expansion valve and return to the suction. It can open and close as many times as needed to achieve a very close control, while at the same time allowing the compressor to run normally.

How do Chillers Work - Expansion Valve Flash Gas

This is the term that describes that portion of the liquid refrigerant which, in any expansion valve, instantly evaporates after it passes through. When this happens, the absorption of latent heat cools the remaining liquid refrigerant in the evaporator to the required saturation temperature.

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Large chiller, refrigerant cylinder, brazing equipment and tools during chiller service company visit

Chiller Service Company Visit

A typical rainy day in the North West, perfect for a chiller service company visit.

News Article No.11

Electrical Faults during Chiller Service Company Visit

This was a return visit to do a refrigerant leak, but the customer alerted our engineer’s attention to an electrical fault. System 2 was found to be locked out in fault on the recent maintenance visit, but now System 1 was being held off too. The fault message on the controller was High Pressure. The controller sends a 240v fault feedback signal, through the high pressure switch which returns to the controller. Our engineer had a look at the wiring diagram to find the number on the terminal strip and checked it out with his multimeter. As he suspected, there were volts going out, but not coming back. He removed the side panel for system 1 and found the switch on the discharge pipe. It was the type that has a red button on the top. When he pressed it, there was a click and volts returned to the controller.

Alarm Reset during Chiller Company Service Visit

He interrogated the Carel controller and followed the reset procedure. The controls went through a timer and then the start sequence was initiated.

Run Testing during Chiller Company Service Visit

After the first scroll compressor started, the head pressure started to build up, but the condenser fans did not start. The on board high pressure gauge carried on rising until the high pressure switch was tripped again.

Head Pressure Control

A transducer on the discharge is used by the controller to sense the pressure in the condenser. When our engineer looked for this in the controller, it was found to be reading wrong by a considerable amount. There is a facility to enter a password and recalibrate the transducer, but this only allows for a small adjustment.

Test Instrument

Our engineers carry various kinds of test instruments which can be used to give a temporary false reading to the controller. This gets the customer up and running and back in production whilst a new transducer is ordered and sent to site.

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Chiller Company Service Low Pressure Visit

Now on with the job to resolve the low pressure issue with the chiller.

Refrigerant Pump Out

Our engineer lifted the liquid line solenoid valve and decanted the remaining refrigerant into a vessel using his recovery unit. He only got out 7kg of a charge of 36kg.

Leak Testing during Chiller Company Service Visit

We use nitrogen for the leak testing as it is an inert gas that will not cause problems with the refrigerant system. There was a sign of the leak in between the steel frame at the middle of the condenser and the condenser tubes. We always strip the chiller down and leak test the entire system though. This is so that the job does not end up going round in circles. At first the leak could not be found, so the pressure was built up in stages, taking into account industry recommended guidelines for a chiller. Sure enough the leak was where it was suspected to be. The occurrence of this kind of leak can be reduced with the use of vibration eliminators.

Brazing during Chiller Company Service Visit

The location of the leak was reported to the maintenance engineer onsite and a hot work permit obtained. The equipment we use is tested at regular intervals to be safe and in good working order. A half hour fire watch was stipulated in the permit, along with the removal of combustible materials from the work location. Correct PPE being donned, he brazed the leak to the required industry standards.

F-gas Pressure Test

A chiller has a different pressure test procedure to other systems, so our engineer built the pressure up according to industry standards, then recorded it on his pressure test certificate. After the required time had elapsed, he rechecked the readings which were found to be satisfactory.

Vac Pump

Each of our engineers carries a state of the art 10 cfm vacuum pump to speed up the dehydration process. We use high quality Torr gauges too, so as to get an accurate pressure reading. A good read back was achieved at the end of the process.

Recharge and Run Test

After the refrigerant recharge was carried out, a satisfactory run test was achieved.

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Refrigerant Economizers

This particular chiller uses an economizer to further subcool the refrigerant. Chiller designers have worked out that the end user can save a considerable amount of money over 10 years if these components are used. After the subcooled liquid refrigerant leaves the condenser, it goes through a plate heat exchanger. Some of this refrigerant, however, is diverted through a thermostatic expansion valve, to the other side of this plate heat exchanger. Further subcooling occurring through the plates.

Expansion Valve

The refrigerant drops in pressure as it goes through the expansion valve. A bulb is fitted to the suction pipe on the outlet of the heat exchanger. The bulb has to be at the correct 'o clock position as oil insulation will affect the operation of the valve. A capillary tube connects the bulb to the valve. Inside the bulb, the same kind of refrigerant that is running in the system is present in its liquid state. As the temperature rises in the suction pipe, this refrigerant boils off, adding pressure into the capillary tube. This added pressure forces the power element down on the valve body and a needle forces the valve open.

Flash Gas

Imagine if the refrigerant was not subcooled at all. It would be around its saturation point with a lot of it flashing off into its vapour phase. Not good when you have warm water coming back from the process. The refrigerant would not absorb very much latent heat into the refrigerant system.

Efficiency

Imagine, on the other hand, the economizer which is fitted to this chiller. Now we have a good proportion of refrigerant in its liquid phase, on the low side of the system, with a minimum amount of flash gas. The warm process water has more chance to cool and the refrigerant absorbs a lot more latent heat. The chiller achieves set point easier and therefore saves a considerable amount of electricity. With this further subcooling monitored for a while, time for a signature from the customer and another job well done!

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400 kw white chillers with panels open during air cooled chiller maintenance

Air Cooled Chiller Maintenance

A nice day to carry out air cooled chiller maintenance at a new site we have taken over in the South East.

News Article No.6

Our engineer attended site at around 9am with the risk assessment method statement having been sent in advance. A site survey was carried out to see if there were any additional risks. Should there have been any changes- the RAMS have a section for the additional risks and control measures. After gaining a permit to work, our engineer was issued with a security pass to access the chiller compound. Three chillers are located in the compound which feed air handlers for a critical application. Two of the chillers are multiple system, scroll compressor, air cooled chillers. The other is a single system screw chiller.

Program Settings during Air Cooled Chiller Maintenance

A complete download of the program settings is available in our engineer’s phone. This is to cross reference the settings, should one of them be accidentally changed by the maintenance engineers. On site engineers are the first port of call for chiller trip outs, with the responsibility to get the plant up and running. We offer real time assistance, over the phone from our Technical Support Desk and can send user manuals in PDF form, direct to their computer. The settings were found to be nominal, so a detailed analysis of the alarm history was carried out:

Alarm History during Air Cooled Chiller Maintenance

In reverse date order, the alarm history of all the systems was interrogated. There had been several system shut downs to carry out the periodic maintenance by the onsite personnel. The electricity having been shut down, there was a subsequent oil pre heating timer in the history too. On Chiller 2, System 1 however, there had been several low pressure trip outs. Our engineer decided to start the maintenance with this system by carrying out a full diagnosis of the low side of the refrigerant system:

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Superheat during Air Cooled Chiller Maintenance

The system runs on R410a refrigerant. This refrigerant has higher operating pressures in comparison with other HFC refrigerants. It has an efficient temperature range which can be seen on a pressure enthalpy chart. Below or above this range- the refrigerant loses efficiency and so has a lower coefficient of performance. The most common saturation point for this refrigerant is 0°C which corresponds to a 7 bar suction pressure in the evaporator. Above this is the superheat of the refrigerant returning to the compressor. On this occasion there was found to be 26°C of superheat and a suction pressure of 4 bar- close to the low pressure trip out. After careful diagnosis, our engineer decided to focus his attention on the expansion valve:

Thermostatic Expansion Valve

There are 4 forces acting on a TEV:

Liquid line pressure coming from the condenser.
Versus
Suction pressure down the equalising line from the far side of the evaporator. This compensates for the pressure drop across the evaporator and shows the true compressor side pressure.

Spring pressure acting upwards and closing the valve.
Versus
Bulb pressure forcing the valve open.

To reduce the superheat, the bulb should have forced the valve open. The refrigerant charge in the bulb acts upon the bellows to achieve this. The reason for the malfunction, on this occasion, was found to be the failure of the expansion valve orifice. It had become jammed- causing a shortage of refrigerant in the evaporator and high superheat.

Latent Heat

Our engineer was carrying out the above fault finding with one compressor running and the other two being held off. This was to prevent a low pressure trip. Where chillers are left running with a high superheat condition, the reduced amount of latent heat causes a higher cost in electricity relative to refrigeration effect (COP) The refrigerant carries on superheating without absorbing latent heat- pointless and inefficient for a chiller.

Chiller Pump Down

For convenience, this chiller can be pumped down and valved off using the service valves. The evaporator can be worked on after breaking in procedures are carried out. Therefore, we have arranged for this to be carried out before fitting the new expansion valve parts. These chillers also have the ability to pump down the refrigerant on receiving a fault feedback from the electronic leak detector. This is an added measure to lower the environmental impact of refrigerant leaks.

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Subcooling during Air Cooled Chiller Maintenance

This is cooling the refrigerant vapour down, through the latent heat phase and then subcooling the liquid down further. On System 2 of the same chiller, a subcooling issue was identified. 21 bar/ 36°C saturation was normal for that system as defined by the fan speed controller. Now, the system pressure was higher at 28 bar/ 47°C saturation, so our engineer decided to work out the subcooling. A very high reading of subcooling was recorded at 28°C this was diagnosed to be due to non condensables in the refrigerant:

System Non Condensables

Non condensables are gases that will not condense, such as, air and nitrogen. If nitrogen is not vented properly and a deep vacuum then achieved, the gasses will remain in the refrigerant system. When calculating the subcooling, the readings work out incorrectly due the presence of the gasses. This can lead to false diagnosis. The remedy for the issue was to arrange a full refrigerant decant, pressure testing and dehydration, before charging with new refrigerant.

Efficiency

Having good subcooling values on a refrigerant system is critical to efficiency. Where there is no subcooling- the refrigerant has not fully rejected all the latent heat from the condenser. This can be seen when looking at a PH chart and plotting the pressures and temperatures. This heat remains in the refrigerant and adds to the system along with heat added from the compressor and heat from the process. This is another reason the coefficient of performance is reduced and so incurring increasing electricity costs for the plant.

Economizer

These chillers are also fitted with refrigerant economizers- one for each system. They work by diverting some of the refrigerant from the condenser, through a small expansion valve, then through a plate heat exchanger. The rest of the liquid refrigerant passes on the other side of the plate heat exchanger and so is further subcooled.

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Global chilled water system service near Saint Basil's Cathedral

Global Chilled Water System Service

Moscow Visit

Always good for our engineers to get out of the country to carry out global chilled water system service. He just had a little free time this visit to see Red Square and Saint Basil's Cathedral. Most of the visit was bogged down resolving issues.

Electrical Problems

One of the chillers was pumping down due to electrical problems. One of the solenoids for the evaporator was not opening. All checked out okay with the solenoid coil and valve. Following the wiring diagram led to the relay board. The relay board is responsible to run various components around the compressor. It was not obviously blown and there were no dry soldered joints on the back. Our engineer decided to tighten up the wiring and unplug, then plug all the connectors back in. When he ran the chiller again the problem went away.

Chilled Water System Global Service

As we provide a global service: we cannot get back the day after leaving site. Therefore, after resolving the above issue, our engineer ran the system in local with some of the other systems off to prove it. All was okay. After completing the rest of the service on the chillers, he put the whole of the plant back in Auto to prove it all together.

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Flow Switch Failure during Global Chilled Water System Service

Due to security issues, this one of the most remote plant rooms for a flow switch to fail. The chiller did not stop when a test was carried out. Our engineer did not want to valve off the water system as the air handlers are old and 'dead heading' could cause failure. Instead, he decided to use the Hand/ Off/ Auto switch for the pump. After several attempts of trimming, he got the switch to settle down, then carried out tests to make sure it would not happen again.

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