17 replies to this topic

Dear All,
Good day to you.
We are manufactures of Chemical Injection Skids, in which we are injecting Oxygen Scavenger (Ammonia Bisulfite or equal). The chemical is stored in a pressure vessel whose design condition is full of liquid (3000mm) + 100 mmH2O + Vacuum (-25) mmH20. Working pressure is atm & temperature is ambient (max of 52.3C) Nitrogen is the blanketing gas used & available condition of N2 @ site is 7bar & PCV to be used to controlling N2 to vessel. We will have to provide a PVRV (Pressure Vacuum Relief Valve).

Kindly help me choose set pressure & set vacuum value for PVRV and also PCV set pressure.

Awaiting some expert advices as i am facing difficulties in choosing the same.

[proinwv]

The selection of a set point must include using the following information:

MAWP
MAWV

and the operating band for the vacuum vent, the blanketing valve, the pressure vent(s). In this one must also allow some differential between these bands to insure that there is no interaction between the devices.

[indiana]

VVS,
Any specific reason to use Nitrogen blanketing for tanks in Oxygen scavenger service? If not, what can be the effect?

[simadri]

VVS

The discharge pressure of PCV should be 25mm WC or 50 mm WC, not more than that. As the only job of nitrogen is blanketing, better to provide PCV discharge pressure as 25 mm WC as your design pressure is 100 mmWC. You should provide a compound transmitter in the tank for pressure and vacuum reading.

[proinwv]

With a PCV setting of 25mm (1"wc) you have the situation that the reseat is then 12mm (0.5") because most simple relief vents require 50% to reseat.

This means that the blanketing set point needs to be near atmospheric to account for the usual +/- 0.5" operating band about set point of these products.

Therefore you have a system operating band for the blanketing that can droop off into the vacuum range or rise to the vent operating band.

[simadri]

proinwv

We can keep PCV discharge pressure as 25 mm WC, but PVRV set pressure for vacuum should be around 75 mm WC.Hence reseating pressure will be above PCV discharge pressure.

VVS

Generally for equipment with PVRV, design vacuum for the equipment considered as (-) 50 mm WC and set vacuum of PVRV used is (-) 25 mm WC

[fallah]

proinwv

We can keep PCV discharge pressure as 25 mm WC, but PVRV set pressure for vacuum should be around 75 mm WC.Hence reseating pressure will be above PCV discharge pressure.

VVS

Generally for equipment with PVRV, design vacuum for the equipment considered as (-) 50 mm WC and set vacuum of PVRV used is (-) 25 mm WC

simadri,

Set pressure for vacuum around 75 mm WC? Isn't it strange?

Also, why is the design vacuum of equipment with PVRV considered -50 mmWC?
In fact, design vacuum of an equipment is independent of having or not having PVRV and depends on its mechanical design.

Fallah

[fallah]

This means that the blanketing set point needs to be near atmospheric to account for the usual +/- 0.5" operating band about set point of these products.

Paul,

As far as i know operating band for a tank blanketing valve (pad valve) mostly is between "lock-up" pressure and "drooping" pressure (with a set point inbetween) normally are provided by manufacturer and might be different for each manufacturer. Hence, generalizing the +/- 0.5" around the set point as operating band for all blanketing valves might not be logical.

Fallah

[proinwv]

I am not generalizing. It is the quoted standard for a major manufacturer of these valves.

Further, having several patents on blanketing valves I can assure you that (dependent upon sizing) the controlled pressure can even exceed a droop of more than 0.5" wc.

As to pressure rise, generally it occurs around lock up (shut off) and is less than 0.5" wc but it can be that much and will increase with aging of the seat seal.

Then there are some pressure regulators that exhibit a boost in controlled pressure as flow increases.

Further, most manufacturers do not test for these variations for all range springs and inlet pressures because of cost of testing and because PRV's are not taken as serious control devices.

Good luck!

[fallah]

I can assure you that (dependent upon sizing) the controlled pressure can even exceed a droop of more than 0.5" wc.

Further, most manufacturers do not test for these variations for all range springs and inlet pressures because of cost of testing and because PRV's are not taken as serious control devices.

Paul,

Thanks a lot for your explanations...but:

In the first statement of above, you might mean ...decrease...not exceed. Am i right?

Indeed, due to the second statement of above can we always rely on giving +/-0.5" around the set point for operating band of a blanketing valve?

Fallah

[proinwv]

Well I stand by my statement, but it does require more explanation. The droop (offset from setpoint) can exceed that amount. That means that the setpoint can decrease more than the stated amount.

Do not rely on this +/- 0.5" for all situations. It is effected by:

The valve style (PRV, PPRV, balanced PRV)
The valve manufacturer
The amount of flow (in percent of full capacity)
The statement by the particular manufacturer.

[simadri]

Fallah

First of all sorry for the typographical error. I want to mean set pressure (not vacuum) for PVRV around 75 mm WC.

Yes, design vacuum of the tank is not dependent on PVRV set pressure. I mean to say in tanks with PVRV, where low vacuum may be created in a tank (only due to failure of air/nitrogen ingress to the tank at the time of discharging), better to keep the design vacuum of tank as (-) 50 mm WC in stead of (-) 25 mm WC and we can set PVRV vacuum at (-) 25 mm WC. This is the way we do.

If the design vacuum of the tank is (-) 25 mm WC, then what should be set vacuum of PVRV as it will require some time to open fully?

[proinwv]

You should also look at your vacuum vent suppliers. Find the vacuum over (under) setpoint necessary to open the vent to the necessary flowrate should it be called on to protect the tank.

You may find it difficult or expensive to obtain a vent that will open under those conditions if the tank is rated at -1" (25mm)wc.

[fallah]

better to keep the design vacuum of tank as (-) 50 mm WC in stead of (-) 25 mm WC and we can set PVRV vacuum at (-) 25 mm WC. This is the way we do.

If the design vacuum of the tank is (-) 25 mm WC, then what should be set vacuum of PVRV as it will require some time to open fully?

simadri,

Design vacuum of the tank shall cover maximum vacuum might be created among various failures and upsets. Hence, you can keep the design vacuum of the tank on (-) 50 mm WC only when the tank has already been mechanically designed based on this value (or higher).

About your question: If design vacuum of the tank is (-) 25 mm WC, then this value minus "undervacuum" (in contrast to overpressure) needed the valve to be fully opened, should be the set point of the vacuum part of relevant PVRV. For example: if undervacuum percent of the valve to be 10% of the set vacuum, the PVRV set vacuum should be around (-) 22.7 mm WC.
Of course, vendor will certainly consider the undervacuum's value in his proposed valve's set vacuum as per your reported design vacuum and necessary flowrate due to vacuum vent.

Fallah

[simadri]

Fallah,

Thank you for your valuable suggestion. But as a process engineer, how will I decide the % of undervacuum? Codes talk about overpressure, but not undervacuum. For PSV, API-520-1 states about overpressure, but API-2000 for PVRV has not taken tha part into account. So if you have any literature/guideline/practical knowledge for PVRV taking care of overpressure and undervacuum, please share with us.

[fallah]

simadri,

Overpressure/undervacuum of a PVRV shall be specified by vendors and it may be different values for various vendors.

Fallah

[simadri]

Fallah

But the process engineer should specify the set pressure and vacuum. How will he set the values even though he knows design pressure and vacuum?

[fallah]

But the process engineer should specify the set pressure and vacuum. How will he set the values even though he knows design pressure and vacuum?

simadri,

Yes, the process enginner can specify the set pressure and vacuum of the PVRV in relevant data sheet. Obviously, those vendors who cannot adjust their products with these set pressure/vacuum values since relieving pressure/vacuum (set pressure+overpressure/set vacuum+undervacuum) would pass the design pressure/vacuum of the tank, cannot submit any proposal to the purchaser.
On the other hand, if due to this limitation, vendors cannot submit proper proposal purchaser have to change the set pressure/vacuum to be able to meet the closer conditions among all vendors conditions.

Fallah