8 replies to this topic

[zavtranguyen]

Dear experts, for a steam system, we often see a steam network of a petro-chemical plant has different levels of opeation pressure , the higher pressure steam network is letdown through  valves or extracted from steam turbines (extraction type) in order to suppy more steam to lower pressure steam networks if required.

 

We can take a simple case: given 2 steam networks one is superheated high-pressure steam (HP, 38 barg, 385 deg.C) and the second is superheated medium-pressure steam (MS, 29 barg, 340 deg.C). Steam goes down from HP to MS through 2 ways: a PIC letdown valve and extraction from a steam turbine (extraction condensing type, driven by HP steam). All steam from MS is taken from HP with a stable amount.

 

In normal operation we has to control and keep the PIC closed, it means all steam from MS is received from extraction steam of the steam turbine. The question is if we let the PIC opened 15-20% (total  HP steam flow rate supplied to MS isn't changed), then we adjust the turbine extraction flow in order to the PIC closed 0%.

How to do a evaluation assessment to know how much profit can be get if we keep the PIC closed 0% in the comparison with 15-20% opening.

 

Could you pls to share how to approach the above evaluation, or any document on practice you have done, please, i looking forward to your comments.

Thank you so much

Nguyen

[Bobby Strain]

Just compare the fuel cost to generate the high pressure steam.

 

Bobby

[breizh]

Hi,

Consider the links underneath:

https://www.tandfonl...668680309509016

https://www.energy.g...ogenerators.pdf

 

In addition:

Calculators 

https://www4.eere.en...amo_steam_tool/

 

 

Good luck

 

Breizh 

[zavtranguyen]

Bobby Strain, on 08 Oct 2022 - 04:24 AM, said:

Just compare the fuel cost to generate the high pressure steam.

 

Bobby

Yes, thank you sir, basically it shoud be done by the way you said, just in some cases when the whole steam network is complexible and the evaluation is for the consuming points which is very far from the boiler, plus steam letdowns through the valve with a small amount.

[zavtranguyen]

breizh, on 08 Oct 2022 - 04:40 AM, said:

Hi,

Consider the links underneath:

https://www.tandfonl...668680309509016

https://www.energy.g...ogenerators.pdf

 

In addition:

Calculators 

https://www4.eere.en...amo_steam_tool/

 

 

Good luck

 

Breizh 

Yes, thank you for you attached with great papers and sites

Regards

[Bobby Strain]

Don't be distracted by anything but the high pressure steam boilers. Their fuel is the only cost element to consider. If you letdown high pressure steam through a valve and not via turbine extraction, you must generate more high pressure steam. Thus using more boiler fuel.

 

Bobby

[zavtranguyen]

Bobby Strain, on 09 Oct 2022 - 04:04 AM, said:

Don't be distracted by anything but the high pressure steam boilers. Their fuel is the only cost element to consider. If you letdown high pressure steam through a valve and not via turbine extraction, you must generate more high pressure steam. Thus using more boiler fuel.

 

Bobby

Dear Sir, thank you for you comments,

Howerver, following the text is quoted from the above attached by Mr.Breizh:

 

" STEAM TURBINE VERSUS PRESSURE REDUCING VALVE OPERATION

 

A question arising frequently in steam systems relates to the benefits  and  drawbacks  associated  with  passing  steam  through  a  pressure reducing  valve  or  a  steam  turbine  to  supply  a  low  pressure  steam  demand. The most appropriate analysis of the economic benefits of operating the steam turbine utilizes an incremental systems approach. The information of primary importance to the analysis is:

• Incremental electric cost

• Incremental fuel cost

• Boiler efficiency

• Steam  turbine  efficiency  (or  the  properties  of  steam  entering  and

exiting the turbine)

• Steam flow rate (or process demand)

 

The term “incremental electric cost” relates to the rate structure or tariff applied to electrical purchases at a facility. In particular, the actual

economic  impact  of  any  change  in  electrical  consumption  is  the  incremental  cost.  Many  times  the  price  of  electricity  is  dependent  on  the amount  of  electricity  consumed,  the  rate  of  electrical  consumption,  as well  as  the  time  of  use.  Most  electrical  tariffs  for  industrial  sites  carry fixed charges, which do not change with respect to electrical consumption. A change in electrical demand will typically not incur the “average” electric cost for a facility but the “incremental” electric cost.

 

To compare the operation of a steam turbine to a pressure-reducing valve, an example is investigated. The example focuses on a boiler producing high-pressure steam, which is operating in support of a site that demands low-pressure steam and shaft power (or electricity).

The investigation  considers  a  facility  capable  of  operating  under  two  different scenarios.

 

In the first operating scenario, the system receives fuel to produce  high-pressure  steam.  The  high-pressure  steam  passes  through  a

pressure-reducing valve to supply the site’s low-pressure steam demand. Electricity is purchased to meet the site electrical power demand. In this scenario, both fuel and electricity are purchased to support the activitiesof the site.

 

In the second scenario, fuel is also consumed to produce high pressure steam; this high-pressure steam is passed through a steam turbine  to  produce  shaft  power.  The  turbine  is  connected  to  an  electric power generator, which supplies a portion of the site’s electrical demand. Low-pressure steam is exhausted from the turbine and is utilized in site operations. In this scenario, fuel is the only purchased utility for the site.

The example system consists of a boiler, a low-pressure steam demand  and  a  steam  pressure  reduction  component  (pressure  reducing valve or turbine). The steam turbine drives an electric generator. Figure 1 is a simple schematic of the system.

https://postimg.cc/XBL7H1r1

 

 

The analysis procedure first determines the cost of fuel supplied to the facility when the pressure-reducing valve is in operation. Electricity

purchased  during  this  operating  mode  is  not  considered  until  the amount of electricity produced through the turbine-generator set is determined  in  the  second  scenario.  The  second  analysis  scenario  determines the cost of fuel supplied to the boiler when the turbine is operating  and  allows  the  electricity  produced  in  the  generator  to  reduce  the total site electrical consumption.

 

One  primary  factor  is  held  constant  in  the  analysis;  the  thermal energy supplied to the low-pressure steam demand. This is not to say the steam flow rate supplied to the steam demand is equal in both cases. In fact, the steam mass flow rate will change because the turbine converts some  of  the  steam’s  thermal  energy  into  shaft  energy.  Therefore,  the steam exiting the turbine will have a reduced energy content when compared to the steam exiting the pressure reducing valve. As a result, when the  turbine  is  operating,  the  mass  flow  rate  of  steam  must  increase  to supply  the  same  thermal  energy  to  the  steam  demand.  This  additional steam flow is provided by the boiler, which requires additional fuel.

 

............

............

Many times the electric power savings ($112,580/yr) is reported as the potential savings associated with operating the steam turbine because this is the avoided electrical purchase. However, recall the fact that fuel consumption increased to maintain the thermal energy supply to the site.

This resulted in an increased fuel consumption of $43,050/yr. Therefore, the actual savings potential is $69,530/yr or approximately $70,000/yr."

 

 

In my case, we use a steam turbine-compressor instead of the turbine-generator. And right now my turbine is operating with 110% designed load and over, so any effort to take steam extracted from it without any letdown valve's support, it means if we try to keep the letdown valve closed, the turbine would be overburdended to play 2 roles: to drive the compressor with a certain power and to supply extraction steam for lower steam system.

 

Thank you, hope to see more comments from Sir and all of you experts,

Regards,

Nguyen

Edited by zavtranguyen, 09 October 2022 - 09:23 AM.

[Bobby Strain]

All that does not apply to an existing system. It is necessary only when designing a new facility. Check with the turbine manufacturer to get the capacity. Turbines are often capable of delivering significantly more than design power.

 

Bobby

Edited by Bobby Strain, 09 October 2022 - 11:16 AM.

[Saml]

You are mixing two cases:

 

A.) The one you have: You have a condensation turbine (condensation can change) and you have a turbine demanding a constant power commanded by the process. All other steam requirements remain the same. In this case, by passing steam thru the turbine, you are reducing the heat to the boiler and the condensation load in the surface condenser.  The only thing you should care about to analyze the benefits is the fuel to the boiler.

 

B.) Your heat demand is constant including (if there is one) the surface condenser load and you are replacing electricity. In that case, you should consider the electricity too, and it is what is mentioned in the publication you cite.

Edited by Saml, 09 October 2022 - 05:50 PM.