6 replies to this topic

I am trying to select the adequate control of two turbo-expander for natural gas working in parallel. Anybody knows the different type of controls? and their advantages/disadvantages ?

Please provide internet sites for reference if any.

Thanks

[wojtar]

I am trying to select the adequate control of two turbo-expander for natural gas working in parallel. Anybody knows the different type of controls? and their advantages/disadvantages ?

Please provide internet sites for reference if any.

Thanks

I'll be dealing with it soon bacause I have to analyse offer for such turbo expander. I'll try to give you some basic concepts when I'm back in the office. There is a very thick book called "Pump user handbook" - many authors. I belive something may be found there. Normally piping and instrumentation diagram (P&ID) for such compressor, where you can see compressor stages, intercoolers, compressor and turbine control scheme, is up to supplier. Such P&ID is not so easy and full of controls, interlocks etc...but if it's just school work maybe could be simpyfied.

Edited by wojtar, 01 May 2010 - 06:05 AM.

[breizh]

Hi ,
Did you try Google ?
This US patent may give you some indication
http://www.freepaten...om/4281970.html

Hope this helps
Breizh

[Zauberberg]

I assume you have 2x50% expanders in the field, so there has to be the Master Pressure Controller/Load Sharing System that will distribute loads between the two expanders, as equal as practically possible.

The Turboexpander-compressor discharge pressure signal is transmitted to the Master Pressure Controller, and the MPC then makes necessary adjustments to:

a.) Expander guide vane position,
b.) J-T valve position,
c.) Anti-surge control for the compressor side of the expander/compressor unit.

Basically, this is the simplified control narrative for all turboexpander facilities I came across. Not sure if your question refers to controlling both expanders at the same time (2x50%) or what possible control schemes we have in general for turboexpanders. You may expand a little bit on that.

Edited by Zauberberg, 02 May 2010 - 05:19 AM.

[Zauberberg]

You may also find this quotation from the GPSA Databook useful:

The expander speed is established by the manufacturer, given the process conditions. For maximum efficiency, the expander manufacturer determines the wheel diameter and specific speed. As plant operating conditions change, the expander speed may change. Fig. 13-75 shows the change in efficiency as a function of change in design flow rate.

Gas entering the expander is directed by adjustable nozzles into the impeller. About one-half of the pressure drop across the expander takes place in the nozzles, imparting kinetic energy to the gas which is converted to shaft horsepower by the expander wheel. Pressure reductions are normally limited to 3-4 ratios. Greater ratios reduce expander efficiency to the extent that 2-stage expansion may be advisable.
The adjustable nozzles function as pressure control valves. A pneumatic operator takes a split range signal (3 to 9 psi) to stroke the nozzles. On increasing flow beyond the full open nozzle position, a 9 to 15 psi signal from a pressure controller opens a bypass control valve. This valve is called the J-T (Joule-Thomson) valve.

Thanks all for your input but what I am looking is :

1- Identifying the different type of controls for 2X 50% turbo-expanders.

From Zauberberg answer I can see that the expander can be controlled based on the pressure at the turbo-expander discharge.

I have also found that it can be controlled based on the pressure at the expander inlet, ect...

Is it only the type of controls used?

2- How to select among them?


Thanks again

[Zauberberg]

It all depends on your particular application. For example, in one LNG plant where I used to work, the pressure upstream of expander was the controlled variable because steady condensation of LNG stream at given temperature had to be achieved (no deviations on upstream pressure), so the pressure downstream was slightly floating - depending on actual capacity of Methane compressors.

On the other hand, some NGL recovery plants (Ortloff's SCORE for example) control the pressure downstream of expander, in the De-C2 tower. Once when you know what do you want to control, it's relatively easy to setup the control scheme.

Best regards,