9 replies to this topic

[morvenlight]

hi,

if a plant is designed on 30MMSCFD and its turn down capacity is 30 percent...then what will happen if we operate the plant below its turn down capacity...

[Erwin APRIANDI]

Working on an engineering company, who is workin to design and construct the plant.
What I can say is, if we design the plant to be able to operate on certain amount of minimum feed flow (turndown) let say 10 MMSCFD, it means that it can only operate at that certain feed flow (10 MMSCFD) and can not go less than that.

This usually, because one of the equipment or system can only be able to work minimum at that flow (10 MMSCFD). And if it goes lower there may be a problem in operation which may damage the system or equipment.

The minimum flow is ussually requested by the operator based on their expected min flow during operation, and it has been requested by the EPC company or consultant to the vendors, so that the vendors will design the systems or equipments accordingly.

However, during brownfield project you may check the actual minimum flow of the equipment or system by checking each of equipment or system mechanical datasheet. Because on reality, the minimum flow that vendor has considered is usually even lesser than what has been specified, which is an advantage for later stage of plant lifecycle.

Edited by erwin.apriandi, 22 November 2011 - 02:22 AM.

[ankur2061]

morvenlight,

I don't think that it may have too much of an adverse impact if you are talking of a gas processing plant of 30 MMSCFD capacity.

Let us analyze this as the movement of material into and out of a system of a fixed volume. When you reduce the throughput (capacity), each molecule has to stay in the system for a longer period of time then it was at the design capacity. In chemical engineering terms the residence time for the system components increases. Increased residence time may be detrimental if the components are not supposed to be in the system for a longer time for reasons of longer exposure to conditions like elevated temperatures and pressures.

As a different example, in plants which process polymers under high pressure and temperature, increased residence time due to lower throughput can severely impact the quality of the final polymer due to effects like thermal degradation because of longer exposure to heat and pressure.

Hope this explanation helps.

Regards,
Ankur.

[fallah]

what will happen if we operate the plant below its turn down capacity...

morvenlight,

Suppose you are thinking about a continuous process!

You will be faced with a product not having the prespecified characteristics and specifications due to following issues up to passing critical limitations:

- Increasing the reaction time if there would be a reactor in the process flow diagram
- Decreasing heat transfer equipment performance
- Decreasing performance of physical separation equipment
- Deposition of suspended materials in piping system due to fluid velocity reduction, if such materials would be applied
- Minimum flow for pumps and surge conditions for compressors
- Parts of piping system may not remain filled with fluid due to fluid velocity reduction
- .......................................................................................
Obviously, there may be one issue/part among above issues that would have/create highest turndown capacity and dictates its own turndown to whole other various plant issues/parts with lower/equal turndown capacity.

Hope above helps you out!

Fallah

Edited by fallah, 22 November 2011 - 02:51 AM.

[morvenlight]

thanks to all of you,

well i am working on a MDEA based gas sweetening plant having a turn down capacity of 8MMSCFD...but a year back due to some reasons we have operated the plant at 2 MMSCFD for about a week....but we have'nt faced any problem with such a reduced flow...that's why i was a bit confused about the turn down capacity...we have observed that even at such low gas flow. all the parameters ramained normal and the outlet gas specifications were according to requirements...

[kkala]

Below, delayed due to temporary web unavailability, speaks generally. Much belongs on the specific unit.
Let us take the example of a fuel oil boiler duty specification, elaborated twice and realized by the successful bidder (without some objection or change). Following was written in this document.
Continuous boiler capacity: A ton/h of steam (of pressure and temperature as required).
Max capacity for two hour operation: 110%*A ton/h
Minimum continuous capacity with all burners in operation: 25%*A ton/h
Turn down ratio for each burner : 4.4 : 1
Note: The latter results from 110/25=4.4
There are burners that can obtain a turn down ration of 10:1 with proper instrumentation and accessories, costing higher and judged to result in lower minimum continuous capacity. In case that some of the burners are put out (placed out of operation), capacity lower than 25%*A can be realized. This would not be recommended for long term use, due to uneven heat load, probably causing higher maintenance (imagine one burner operating instead of two). But it does occur a few times in practice, without apparent consequences.
Refinery units are usually designed for a minimum capacity of 30% - 50% of the normal one, understood to represent continuous operating conditions, realizable by vendor's (licensor or contractor) instructions. But we can see, extending the above, that capacity can go lower than what vendor defines, if we define a relevant procedure. This can be an easy one (e.g. recycling), or requiring a complete consideration of capacity limitation for each piece of equipment, a reverse "debottlenecking" needing even plant modifications.
Operation at such low capacities can have some risk (if something is not taken into account), apart from loss of material (efficiency goes lower and lower if nominal capacity goes down, even when it is above vendor's minimum). It has to be done, if necessary. In such cases shutdown procedure might be useful (not always, only in some cases), reaching capacity reduction to 0 %.

[fallah]

morvenlight,

IMO, turn down capacity would usually be considered based on long term conditions such that short term operating below that capacity may result in no immediate problem but you will see the problem in, let say, reduction of plant life time.

Nonetheless, in your case either it might be considered the big safety margin value between given turn down and the critical capacity or it might be calculated based on considering some remote conditions among probable ones.

Fallah

[morvenlight]

thanks all,,

well what about minimum operting pressure?

if we operate the plant below minimum operating pressure on a continuous basis....what will be the effects on plant?

[fallah]

morvenlight,

- Malfunction of reactors and separation systems (e.g. distillation towers) need to be operated at a minimum pressure in order to have a prespecified performance

- Malfunction of the plant in hydraulic point of view

- Possibility of damage due to external pressure for systems/equipment operating below atmospheric pressure

-..................................................

Fallah

[Erwin APRIANDI]

Dear Morvenlight,

A plant will consist of many equipments and integrated system which, you can reduce the minimum operating pressure of the overall plant but you may experience less performance of some equipments or system.
For example in a separator, if you operate this on less pressure but at same flowrate (mass) I can assure you that you would not get the separation that you wanted, it is because of the volumetric flowrate of the feed is increase, and the cross sectional area inside the separator or separation efficiency in vessel internal is decrease.
I have done a project of adequacy check, in order to ensure is there any additional facility required just because the operating pressure of the plant is decrease.