suction pressures

[steven f.]

Just reading through the forums and noticed you talk about cracking a holding tank during a pump-out. Just how much suction is used during a pump-out? Is there some type of in-line device to prevent too much suction on my tank should an unknown failure occur in my tank vent? What about flexable tanks, can they be injured from a pump-out or will they collapse? Not terribly important questions but one's I'm curious about just the same.

 

[Peggie Hall/Head Mistress]

Pumpout suction varies....

But even standard 5/8" tank vents allow enough air into the tank to prevent even the strongest pumpout from pulling a vacuum that can result in a cracked tank. There is no device that can prevent it...only an attention span. The potential for a cracked tank isn't the only issue. When the tank vent becomes blocked, flushing the toilet pressurizes the tank, which can also crack the tank...unless it blows out a fitting first. The least you can expect is a sewage geyser when the cap is removed from the deck pumpout fitting. The article "Tank Vent Maintenance" in the Head Mistress forum reference library (click on the link to it on the forum homepage) explains the problems and how to prevent them in detail.

 

[Bill O'Donovan]

Really

I can't believe there would be enough suction to implode the tank. Recalls the famous line by Willie Nelson in an old movie. He was asked what he was going to do that night and replied, "I'm gonna go find me a gal who can suck the chrome off a trailer hitch."

 

[Peggie Hall/Head Mistress]

Believe it, Bill...it happens

An overboard macerator isn't likely to do it--it just won't pump anything out of the tank once it pulls a vacuum...but most pumpouts today ARE that strong. The tank doesn't actually implode, but when enough suction is applied against a vacuum, the walls are pulled in until one cracks. Even the toilet can do it if the tank becomes pressurized enough to push the walls out. If you go back a couple of weeks, you'll even a find a story about a stainless tank that split at a seam because the owner didn't have sense enough to stop pumping the toilet against--by his own admission--a LOT of backpressure. Keeping vents, especially the holding tank vent, is VERY important.

 

[Carl lebo]

absolutely

Remember, all the pump has to do is pull a small amount of vacuum when atmospheric pressure (14.7 psi) pushes from all sides on the outside and your done. (Having done the same to a heating expansion tank in a house when the vent hole was plugged, I certainly believe it!)