Gas Drying in Romania

 

When state-owned Romgaz,  Romania's leading supplier of natural gas, contracted German engineering company BIS E.M.S. to build twelve new gas drying plants it was buying into thirty years of specialised experience.

E.M.S. (part of the Bilfinger Industrial Services group) has a long and successful track record in gas engineering, but it faced new challenges on this €24 million project - the company's first-ever turnkey operation.  Operational requirements were particularly demanding, prompting E.M.S. as main contractor to find new technical solutions, notably in its adoption, for the first time, of Hydra-Cell  pumping technology.

Gas dehydration is a continuous process, operating 24/7 throughout the Romanian winter.  At final stage, water vapour in the rising gas stream is absorbed by dissolving it in an opposing downward stream of tri-ethylene glycol (TEG). Water-rich glycol leaving the bottom of the vertical absorber vessel is then regenerated by ‘boiling out' absorbed water, recovered and re-used.

The pumping system delivering TEG to the top of the absorber vessel is a critical dynamic element at this stage, in which wet gas, from underground storage or directly extracted from wells, finally reaches a dew point of -15ºC at 40 bar.  Dehydration will not reach that level of efficiency unless the glycol stream is delivered accurately and reliably.  Each drying installation therefore includes a second TEG pump on standby, with provision for automatic switchover.

The flow of recovered glycol with a purity of  99% must be calculated  in accordance with the rate of gas flow, which varies across the 12 plant locations from 200 l/hour to 4000 l/hour. Gas pressures also vary between 10 barg and 30 barg.  Four of the plants directly service gas extraction lines from underground deposits while the other eight serve three different storage facilities.

Such conditions are more complex than those in Germany, where gas dehydration plants connected to the west European distribution network can be designed for a single operating point, with gas flow, gas temperature and gas pressure (at 70 barg) constant.  In specifying for German conditions, E.M.S. had normally relied on pumps of the piston plunger type, purchased from various well established manufacturers - but all essentially similar in design and construction.  They are generally heavily engineered, and incorporate features such as mechanical stroke adjustment.

Plunger pump limitations

One limitation of this design is that flow capacity options are achieved by changing the internals of a basic pump of fixed size, while (for practical reasons of manufacture) the choice of pump sizes is fairly restricted.  On this project it would have meant that some pumps at least would be overrated for the flow and pressure they had to deliver.  The customer would be paying for a significant amount of extra metal - adding not only to initial purchase cost but also to spares and other costs.

Another issue of concern to E.M.S. was the perceived noise level and vibration associated with conventional  piston plunger pumps. On this contract and other new projects. E.M.S. looked to reduce measured decibel ratings from 85 decibels to 65.  Staying with conventional pumps, the only way improvements could be made was by means of shrouds and sound deadening materials, adding cost and bulk to already massive units.

A new pumping alternative was needed.  The company found it in the Hydra-Cell pump, introduced to E.M.S. by specialist supplier Verder Deutschland.  Its unique seal-less design offered real advantages in terms of flexible performance, compact size, perceived noise reduction and lower life cycle costs.   Combining these benefits with accuracy and established reliability,  the Hydra-Cell concept enabled E.M.S. to explore the possibilities of a more progressive pumping solution.   The pumps were eventually specified for pumping TEG in each of the twelve new dehydration plants.

Using Hydra-Cell pumps addressed a number of issues.

By exploiting  their wide ranging flow capabilities, only three standard models were needed to match precise operating requirements across all twelve sites.  These pumps, the G03, G10 and G25, each rated for working at pressures up to 70 bar, could deliver controllable flows ranging from 10% up to 100% of their rated flow.

Whereas the typical piston-plunger pump/motor unit might have a 2m x 2m footprint and occupy 1.5m in height, the largest Hydra-Cell pumping unit (including motor) installed on the project requires a ground space no bigger than 1m x 0.3m, with an overall height of approx 0.3m.

Space saving of this order was made possible by the simple build of the pump, its energy efficiency (enabling a relatively small motor to be fitted) and its design in which three hydraulically balanced diaphragms are combined in a single pump head.  This arrangement also makes possible the short-stroking high-frequency pumping action and smooth, low pulsation delivery which helps to reduce perceived sound level.  No shrouds or sound absorbers have been needed on the Hydra-Cell units for the Romgaz sites.

With no dynamic seals or packings, Hydra-Cell technology is low maintenance and the pumps can also dry-run indefinitely.  (Seal wear in piston pumps can add substantially to the frequency of maintenance and the cumulative cost of repair work.)  Apart from routine maintenance and some minor adjustment work following commissioning, all the dehydration plants have run smoothly since start-up - to the general satisfaction of the client.     

E.M.S. report that success on this project led directly to further new-plant or extension contracts in Romania and elsewhere.  Hydra-Cell pumps have been specified on these projects, even in cases where initial specifications called for pumps from other manufacturers.  They have also been retrofitted on an existing site where space is at a premium. 

 

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