VESSEL SIZING - RETENTION TIME 4

[virata]

Hello guys,

I have a question regarding Vessel sizing (an Air Receiver within Air Compressor Package). The vendor got back to us and said that they need to know three things:

1) Retention time
2) Beginning Pressure
3) Ending pressure at the end of retention time (and also flow at this period)

Now, I'm quite confused with this beginning and ending pressure. Are these the same as Inlet and Outlet pressure of the vessel? and how is it affected by the retention time?

Thanks in advance,
Virata

 

[Chemitofreak]

Hi,

First of all please specify the type of Compressor in your package. This is important aspect for specifying the compressor control methodology.

My initial guess is that your compressor is oil free rotary screw compressor.

The oil free rotary screw compressors are generally operated by Load/Unload control.

In this type of control when the upper pressure setting is reached (in the receiver), the pressure switch sends a signal to close a valve at the inlet of the compressor (but maintaining a calibrated low flow), reducing the mass flow through the compressor. Simultaneously a blow-down valve, installed in a line coming from the compressor discharge but prior to a discharge check valve, is opened. When the blow-down valve is opened, the compressor air end discharge pressure is lowered gradually and the discharge check valve prevents back flow from the system or receiver. Vice versa when the lower pressure is reached in the receiver.

The retention time to my knowledge is the rescidence time of the receiver, the begining pressure and the end pressure is the pressure where you want to load and unload the compressors respectively.

Revert in case of any query.

Dinesh S SHELATKAR
Process Engineer

 

[virata]

Hello Dinesh,

Compressor is Oil-lubricated screw compressor and is operated by Load/Unload controls(or Auto Dual Control).

The vendor actually did assume that the beginning pressure is the same value of the outlet of the compressor discharge and air dryer discharge (dryer in the package), is this assumption correct? What the vendor is asking now is the ending pressure at the end of the retention or residence time.

With your reply, are you saying that end pressure is the (minimum?) pressure to unload the compressor? If so, how is this being calculated or determined?

Thank you,
Virata

 

[EmmanuelTop]

End pressure also refers to the minimum pressure required for proper operation of instrumentation/valves, and below which a PSD is usually initiated.

Retention time is the time required to shutdown the plant safely in case of permanent loss of Instrument Air. Most of the standards set this value at 15 minutes. See the document

http://www.chagalesh.com/snportal/uploads/chagalesh/karafarinan farda/jozveh/process/8.pdf

and page 8 onwards in particular.

Dejan IVANOVIC
Process Engineer, MSChE

 

[TD2K]

THe air receiver and piping provides inventory for the air system. The rate you are consuming compressed air (assuming no makeup from the compressors is occurring), sets how fast the pressure falls.

Let's say your air consumption is 100 scfm and you need the air system to provide that for 3 minutes before you want your compressors to begin to load again. Total air that has to be supplied is 100 scfm * 3 minutes = 300 scf. If the compressors unload at say 105 psig and start loading at 100 psig, you have a buffer of 5 psi,

Now you go back to the old ideal gas equation, PV=nRT. 300 scf = 0.79 lbmoles. Substitute that and you find that the volume you need 220 ft^3. Your piping also provides surge volume so you don't have to have a 220 ft^3 receiver. Air systems typically have too little volume capacity (because it costs money) and as a result, your air compressors are loading and unloading frequently. It's like using a bicycle air pump to pump up a bike tire versus one on a big dump truck. The pressure in the bike tire changes quickly with a fe pumps, not so with the bigger tire because of the much larger volume.

You can set up a spreadsheet and try different numbers and see how they work out.

 

[Chemitofreak]

Loading and unloading of the air compressor is based on the instrument air receiver pressure.

Please note that if you have more than one air compressor, there will be sequential loading/unloading of the compressor based on the decrease/increase of the air pressure.

Let's consider a case where the instrument air min, normal and max pressure shall be 90 psig, 100 psig and 120 psig respectively.

All the compressors will unload when the max pressure is reached i.e. 120 psig.

If the pressure drops below 120 psig, one of the compressor will load, if the pressure is still reducing say 100 psig, another compressor will load, and so on till all the compressors are able to maintain the required instrument air pressure.

These settings vary from project to project and are also based on the instrument air pressure and compressor capacity. Discuss with CLIENT and VENDOR to reach a consensus.

Additional Information:

Generally Plant Air, Instrument Air and Air to Nitrogen package is provided by the same compressor. If the Instrument air pressure starts to drop, the plant air supply is cut-off, if the pressure still decreases the air to nitrogen package is cut-off, if the pressure still decrease then the whole plant will go to a pressurised shutdown.

Revert in case of any query.



Dinesh S SHELATKAR
Process Engineer

 

[rconnor]

It's important to know what the tank is being used for. Some things could be:
- Increase the load cycle time (this decreases the number of starts/hour or allows a load/unload unit to turn off, which saves energy)
- Help keep a stable system pressure as demand goes up and down
- Ride out large demands without dragging down the system pressure (any intermittent, short duration, high flow event)

- This is an efficient alternative to providing more horsepower to satisfy the demand​

- Note that if your doing this, placing the tank close to the demand and using metered refill is important​

- Provide storage in case of compressor outage/power outage (as EmmanuelTop suggested)
- A combination thereof

What the receiver is being used for will impact things. If you're just using it to increase load cycles or help stabilize the system pressure then rule-of-thumbs for storage size range anywhere from 3 to 10 gal of storage per rated flow of the trim compressor. More storage usually means more energy savings (by allowing the unit to shut off between cycles, reducing the time spent unloaded) and greater reliability. If you're using it to ride out large, intermittent demands, then you can use this calculation to size the tank (which can be derived from what TD2K pointed out):
V [gallons] = dQ [scfm] * 7.48 [gallons/ft^3 conversion] * Pa [psia] * dt [minutes] / dP [psi]

Where:
V = size of the receiver, in gallons
dQ = flow into the tank (supply) - flow out of the tank (demand) (note: this will be negative for draw down but so will the dP value)
Pa = atmospheric pressure
dt = the duration, in minutes, of the event or "retention time" or the time between P1 and P2
dP = P2 - P1 (note: this will be negative for draw down)

P1 = the "starting pressure", the pressure of the tank at the start of the event. This value changes over time and so the value to use is dependent on the use. To be safe, it can be the lowest "load" pressure (i.e. the lowest normal pressure, assuming all units can keep up with production). So, in a cascaded pressure band system, it is the load pressure of the last unit to turn on. This is "safe" because it's the lowest possible "normal" pressure before the event starts.

P2 = the "end pressure", the pressure of the tank at the end of the event. Again, this is dependent on the use but is likely the lowest acceptable pressure in the plant or the lowest acceptable pressure for critical end uses.

So, the greater the "retention time" required, the larger the storage will need to be. The smaller the permissible drop in pressure (dP) is, the larger the storage will need to be.

For example, in the event of a loss of supply air (all compressors go down) (cfm in = 0), I need to be able to keep critical operations going for 15 minutes (dt=15). The critical operations (+ leaks) require 50 scfm (cfm out = 50). The last unit loads at 90 psi (P1=90) and the lowest pressure required to operate the critical operations is 75 psi (P2=75).

V = (0-50) * 7.48 * 14.7 * 15 / (75-90) = 5498 gallons.

Now, say that is too big/costly of a tank. You can reduce the required size of the tank by increasing P1, i.e. by pushing the pressure settings of the compressor up, giving a larger pressure differential. Let P1 = 100 psi. Note that this will increase the energy consumption of the compressors. Now, the required storage will be:

V = (0-50) * 7.48 * 14.7 * 15 / (75-100) = 3299 gallons.

 

[chief]

Rule of thumb for discharge pressure approx. 125psi.
Max Flow rate of compressor/s in CFM = Volume of air receiver in gallons.
This volume allows for dwell time and prevents rapid cyling of comressor.

Offshore Engineering&Design

 

[rconnor]

I’d recommend more than 1 gal/full load cfm. I know it’s very common in industry but it’s one of the reasons compressed air systems are such a headache for most places. It leads to very poor control of multiple compressor systems (hunting, system instability) and an inability to handle large, intermittent air demands. From an energy standpoint, you’re spending most of your time either in blow down or unloaded because of the very quick load cycles.

With more storage, you can handle the hand-off between units much more reliably and you can even handle short burst (ex. dryer purge) without needing another unit to turn on. Also, because you’re increasing your load cycle time, you spend less percent of the time in blow down and can even (depending on the controls) allow the unit to completely shut off between cycles.

There’s a big difference in energy savings and system performance between 1 gal/cfm of storage and 5 gal/cfm of storage. You get diminishing returns above 5 gal/cfm up to about 10 gal/cfm.

For VSDs you can use the same numbers, just replace full load cfm with min speed cfm.