Key Features of WHRU’s
Both the above WHRU’s have been designed and supplied by CiTECH. This variant of a Vertical, Integral By Pass Traditional WHRU (on the left) is one of three units installed on a FPSO and the CiBAS WHRU (on the Right) is a single unit installed on a Platform Deck as a modular retrofit to an existing gas turbine package
They both have a thermal duty of 23MW and are recovering this heat from a GE2500+ gas turbine.
The heating medium is water and water glycol respectively, with the traditional WHRU having a higher flow rate but at a lower temperature differential than the CiBAS unit therefore making a valid process comparison.
In this comparison the CiBAS WHRU offers a 20% space and 40% weight saving over the traditional WHRU. It has to be noted in this comparison that the traditional WHRU does not include the support steelwork required to hold the unit up including the silencer for the Gas turbine.
Table showing Key WHRU Features
|Weight, Flange to Flange||70,000kgs||49,000kgs|
|Centre of Gravity (from support level)||11,000mm||7,200mm|
|Deck Footprint||Deck Footprint 31.7m2 (Excluding Support Steelwork)||26.5 m2 (Including Support Steelwork)|
|Height (from support level)||17,500mm||13,000mm|
|Space Envelope||Space Envelope 5507m2 (Excluding Support Steelwork)||3505 m2 (Including Support Steelwork)|
|Assembly Time||1-2 weeks each (Excluding Support Steelwork)||1-2 Days each(Including Support Steelwork)|
|Access Points||2– Heat Exchanger & Dampers||1 -Coil –Sleeve|
|Gas Turbine Silencer||Required||Not Required|
As the traditional WHRU has very little inherent strength to self-support it requires additional support steelwork. This normally is within the scope of work of the EPC (Engineer Procure Construct) Company and can be substantial (as shown below) due to the forces applied to it coupled with the WHRU’s higher weight and Centre of Gravity.
This is the actual supporting steelwork for the Traditional WHRU used in this comparison. There are three WHRU’s in this project with a total weight excluding silencers of approx. 210Tonne but required a support structure of approx. 300Tonne for them to be mounted on.
Based on an estimated cost of £35,000* per tonne for design, procurement, fabrication and installation this support steelwork cost approx. £10,500,000.
*Raw I section steel cost £1,690/tonne
The above shows the CiBAS unit installed on the platform. To allow the installation of the unit the existing gas turbine silencer and stack were removed and minor modifications made to the deck support structure to facilitate the CiBAS unit.
The CiBAS due to its compact lightweight monocoque design is self-supporting and should any supporting steelwork be required for Pitch, Roll & Heave, Tow Out, Wind/Explosion forces it will be considerably less than that used for the Traditional WHRU.
The CiBAS WHRU is normally delivered to site as a single lift cold commissioned unit, whereas the Traditional WHRU will come in a number of sections that will need assembly at site over a period of several days then cold commissioned
The Heat Exchangers shown above are both rated at 23MW Thermal Duty. The Serpentine package (left) is the actual Heat Exchanger from the FPSO WHRU, while the CiBAS Coil (right) is an example from another CiBAS project were hot oil was the medium and the gas turbine is of a different type.
Table showing Actual Key Heat Exchanger Features
|Number of Tube Welds||736||280|
|Number of Header Stub Welds||92||72|
|Pack Size||2 x Bundles = 1 Pack||2 Packs of 8 Coils x 4 turns|
Due to the serpentine nature of the Traditional WHRU heat exchanger the process fluid is potentially subject to a process pressure drop at each 180° bend at the end of each pass. Also due to the straight lengths of tube creating a laminar flow within the heating medium the heat transfer coefficient is lower.
Whereas the helically wound coil of the CiBAS does not have these 180° bends, therefore it can accommodates low process pressure drop requirements. Plus the coil allows a Higher Heat Transfer coefficient due to enhanced turbulence caused by the geometry of the coil bundle.
There are saving advantages in both time and cost with helically wound coil with there being less material required overall but the major savings is in welding and NDT costs.
In both types of heat exchanger each tube and header stub weld is subjected to Radiography NDT, there is at an average cost per weld of £28 for NDT the CiBAS has a saving of £13,300 over the Traditional WHRU.
Exhaust Gas Dampers
The Traditional WHRU will have a set of Multi Louvered Dampers (left), one for the Bypass and one for Duty Heat.
It will be normal for them to be mechanically linked therefore ensuring when one set is open the other set is closed or somewhere in between. The gas turbine will trip if its control system detects no open path for the exhaust flow.
To control the exhaust flow to bypass or duty heat the louvres have to act against the exhaust gas flow. During which, they are subjected to considerable stress forces from heat and turbulence. The most popular gas turbine offshore has at 100% load an exhaust gas flow of 95kgs/sec.
To create 100% sealing efficiency for full bypass operation the multi louvre units will require additional fans & ducting for sealing air, plus Inconel sealing tips on the louvre edges. However over time due to the forces from the exhaust gas flow the sealing efficiency will decline.
The CiBAS controls the exhaust gas with its single moving part the Sleeve (right).
As the sleeve moves in a linear direction it’s not subjected to the turbulent forces from the gas turbine exhaust
The sleeve will Fail Safe into bypass and seal through accumulator pressure, therefore the gas turbine can keep running
Although not having a 100% efficient sealing capability when in full bypass the sealing efficiency of the CiBAS sleeve will remain constant throughout its life