i am faced with a new condition, there are 6 gas turbo compressor ms-5002D split shafts. hp shaft has constant 5100 rpm and lp shaft is variable speed, the minimum speed for lp shaft is 60% (3450 rpm).

when the start sequence comleted, we have 100 % for hp shaft and 60% lp shaft. since last month one of our compressor has this situation: after complition of start sequence 92% for hp saft and 60% for the lp shaft. for reaching the 100% of hp shaft, we need to increase the lp shaft 83% to have the 100 hp shaft (5100) rpm.

i need to find a solution about this problem

 

What have you done to try to troubleshoot the problem?

What kind of turbine control system is being used to control the unit with the problem? (It makes a difference because the type of control system may or may not have some ability to trend variables).

Why do you expect us to know everything about your application, its age, its control system, its auxiliaries, and what you have done or not done to try to troubleshoot the problem? If you can tell us what you've done and what the results were we can skip that and go to something else.

Good luck with your condition.

 

dmi,

Without giving the application type of the turbo compressor such as, for midstream, upstream or boosting station etc, it will be a bit challenging to give you the correct answers.

Since the operational theory of a two shaft GT is a bit complex, I need to know more details about the plant control philosophy such as the compressor control system (CCC?), load sharing system (CCC?), HRSG (if used) control system (DCS) and one of the most important information whether these compressor station are operating in parallel or series or a combination of this?

I will try to give you first the basic control philosophy of a two shaft GT HP speed control. We will assume that the GT has reached complete sequence and ready to be loaded (MK5, DCS and CCC):

Due the fact that the S2N (second stage nozzle) is controlling the HP speed and this control can be programmed in two ways. One way is in simple cycle and the other in combined cycle mode. As per your description, it appears that your system is programmed for simple cycle application (No HRSG used)? If this is the case the GT HP speed is varying between 99 to 100% speed (TNH). In combined cycle mode the HP speed is varying between 92 to 100%.

If you will start the GT in Remote mode (via DCS), the GT will immediately follow the process load reference point (TNRP). This reference point will be same for all your other GT's if they are operating parallel (or some of them).

It may be also different if load sharing is not used or if the GT is in "LOCAL" mode. If you start and load the GT in Local mode, the control system will bring the GT HP speed to minimum operating speed of 92% and LP 60 %( in case of combined cycle programming). As per your information appears that all the GT's were behaving the same during start up (HP speed was about 100% while LP speed is 60%, except for one GT!

For the moment I can only think of two possible reasons for your problem:

1) Due to the operating point of the process compressor (I assume centrifugal compressor), the GT HP speed will or not able to reach the minimum operating speed of 100%. The root cause of this could be due to problems with the process compressor Anti Surge Valve (ASV) problems and or if used load sharing system problems (unlikely).

Depending on the plant control philosophy, sometimes there are multiple process inputs and manual process set points from various systems within the process that will affect the operation of GT. All these parameters are used in the compressor control and DCS system (ASV/LOAD controllers) and interact with each other and the GT control system use the outcome of all these parameters as the Process load reference input.

2) Since you have a "D" type two shaft GT, it looks like that its recently upgraded? This type of GT will give you more output since the firing temperature is higher as its predecessors (B and C types). This upgrade requires also different control parameters. Did somebody evaluate all these parameters? Are they equal for all the GT's? Sometimes this kind of problems starting "popping up" on a later stage when the operator decided to operate the GT in different loading mode(s) and/or if there are process changes which will reflect in GT operating behavior.

Since the S2N is controlling the HP shaft speed and exhaust temperature, you have to focus on the S2N control parameters.
Check the following parameters for all the GTs (for MK5):
TNKRHLSS=
TNKRHHSS=
TNR=
TNRH=
TSRNZ=
TSNZ_POS=
FSR=

Note: Closing the S2N will decrease the HP speed and opening will increase the HP speed.
Open=15.0DGA
Closed=-5.0DGA

Docendo Discumus

 

Dear sir,

thanks for great recommendation. let me explane about Our station.

We have here 6 turbo compressors MS5002D, and controlling through Mark6 and our DCS is yokogawa "CENTUM 3000" and in this situation because we are at start up phase of our refinery we don’t use DCS for controlling our turbine through load sharing system.

These compressors are installed parallel to receive the sweet gas (methane) from separation column at 31 brag and deliver to the gas pipeline at 85barg.

Our compressor is BCL506 and max flow rate is 395 tone/hr
During the first start up from the last October till March our situation after start sequence completion was like this:

HP shaft speed 100% and 5100 rpm.
LP shaft speed 60% and 2800 rpm.
At this moment the compressor is running in recycle mode.

Now new situation after start sequence completion:

HP shaft speed 92and 4692 rpm.
LP shaft speed 60and 2800 rpm.

In this situation we need to increase the LP shaft speed up to 70.1 % to reach 5100 rpm for HP shaft. Also S2N = 15.23 DGA. For exporting 380 t/h by this compressor we need to increase the Lp shaft speed 89.4 and the S2N is 12.23 DGA

best regard
dm.irani

 

I don't see what was unclear about Ore Rotundo's reply. He said, there can be two modes of operation: Simple Cycle and Combined Cycle. From the sounds of it, someone has, inadvertently or not, selected Combined Cycle control. Have you verified whether or not Combined Cycle Control has been enabled or not?

Combined Cycle control mode would be used to maximize exhaust temperature to maximize steam production from an HRSG which the gas turbine is exhausting into at part load operation (part load meaning partial compressor load).

And, what is the importance of maintaining HP speed if you can achieve the compressor speed you require if the HP speed is not at 100%?

So, let us know what you discover about the selected operating mode. And let us know why 100% HP speed is so critical to the operation of your plant. Usually with two-shaft machines, the HP speed is independent of the LP shaft speed, and in depending on the selected mode of operation (simple- or combined cycle) the HP shaft speed will vary, while the output of the unit (LP shaft speed) is adjusted to maintain desired torque production.