Has anyone protected an atmospheric storage tank against insect ingress using screens or PV valves? 1

[CalumW]

Hi all. Was wondering if anyone here had any experience in this type of problem. The company I am working for wishes to protect all of their atmospheric storage tanks against insect ingress. Some of these tanks only vent through the side entry overflow pipe, some of them have a separate side entry overflow and top entry vent pipe, and some of them only have a top entry swan neck vent pipe. My thoughts are to consider the following options:

1) If the tank only has a side entry overflow, modify the pipe so that it includes a vent cap with insect screen at high level, then put a non-return flap valve at the end of the overflow. If the tank has separate overflow and vent lines, put a vent cap with insect screen on the vent pipe and put a non-return flap valve at the end of the overflow. If the tank only has a top entry swan neck vent, then modify this to have another pipe teeing off the vent pipe with a vent cap and insect screen at high level, and install a non-return flap valve on the end of the swan neck vent.

2) If the tank only has a side entry overflow, put an in-line PV valve in the overflow. If the tank has a separate side entry overflow and top entry vent pipe, replace the vent pipe with an end of line PV valve, and put a non-return flap valve on the end of the overflow pipe. If the tank only has a top entry swan neck vent, replace this with a PV valve

With option 1 my concerns would be that if the insect screen got blocked with dust, dirt or insects, the tank would have no vacuum protection thanks to the non-return valves I'd be putting on the overflow lines.

With both options, I am also concerned about putting a flow restriction on the overflow line, either with the non-return flap valve or the PV valve. I'm assuming all the current overflow lines (or vent lines if there is no overflow) have been sized to prevent overpressurisation in the event that the vessel experiences maximum inlet flow and all the other outlets are blocked. I would need to ensure that the overflow pipe was still fit for purpose even with a non-return valve on the end or if the vessel was relieving liquid through a PV valve

If anyone is able to recommend something, or if anyone has seen a similar arrangement, or any other method of protecting a tank from insect ingress that I hadn't considered, then it would be great to hear from you.

 

[LittleInch]

You need to approach this with great care.

~Get it wrong and you can very easily collapse a tank, especially on in breathing / vacuum or over pressure leading to damage or roofs lifting. Max vacuum and over pressure is often limited to low numbers of inches of water column, sorry if that's obvious, but some people don't think of it properly.

API 2000. Buy it. Gives you the required vent sizes. you can't "assume" anything - you're an engineer - design, calculate, check, don't "assume" for something as vital as this.

Many companies offer P&V valves with insect screens - but first find out / calculate the flow you need and allow for 200% open area of the screen as if there is one thing that can happen to a screen is blockage. Normally insect screen is offered as an option.

e.g.

http://www.emerson.com/catalog/en-us/enardo-series-550

http://www.kspc21.com/product/01.php

http://www.opwglobal.com/products/u...essure-vacuum-vents/623v-pressure-vacuum-vent

Search venting and PV valves on this site - there are many such posts offering advise.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jari001]

Are you able to install sonar insect repelling devices around the tanks or spray the immediate area with an insect repellent? The spraying is an option of last resort, since it's a non-permanent solution that will require manpower.

 

[CalumW]

Thanks for the responses. My problem is going to be determining what the maximum and minimum permissible pressures are going to be for each tank, as these are not specified in the tank documentation (often it is specified as 'liquid full' or 'atmospheric plus liquid head'). If I could establish exact values for these, then I would be able to calculate if a PV valve, insect screen or non-return valve could potentially cause the design pressure to be exceeded.

I don't think our client would be willing to accept a solution of using a sonar insect repellent device as it would not be as effective as creating a physical barrier between any insects and the process. Insect repellent may bring about a risk of contaminating the process itself if it is not applied correctly.

 

[LittleInch]

In terms of vapour pressure, you are best (IMO) to take the definition used in API 650.

Positive pressure on "atmospheric" storage tanks should not exceed either 18 kPa / 2.5 psi or the weight of the roof converted to pressure whichever is the LOWER.

Max Vacuum pressure is 1 inch water guage (0.25 kPa) unless otherwise specified and that is probably your key concern. Storage tanks are notorious for collapsing with very low vacuum levels.

I suspect you may find out that your current vents are undersized before you start adding on additional resistances with your PV valves and insect screens. I've never normally see a tank vent through the overflow only. If you start flowing through the overflow you would restrict the air venting and could block it completely. Very poor practice.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[CalumW]

Thanks for referring me to these values, that's a great starting point.

In the case of the tanks that vent only through the overflow, the overflow pipe is generally much larger than each of the inlet pipes (6" compared to 2" in many cases). I believe they have been sized to allow simultaneous air venting and liquid flow but I will need to check.

 

[bimr]

Insect screens are commonly used on water storage tanks:

http://www.draytontank.co.uk/Tank-overflow-kits-fitting-mesh-rodent-screen.htm

"Tank air vents must be fitted with a #24 mesh non-corrodible screen to prevent contamination from entering the water system. A vacuum/pressure relief valve or another mechanism may be needed to protect the tank."

https://www.epa.gov/sites/productio...eys_significant_deficiencies_presentation.pdf

 

[CalumW]

Thanks bimr. I will check to see if a 24 mesh screen is feasible by expanding the vent line so that the flow area is not reduced by the vent (with a 100% margin)