# Constant Pressure Variable Volume Tanks

Do constant-pressure variable-volume tanks exist in industry? I've read about water-column and membrane-based designs, do either of these exist as OTS components, and where would one get them?

Edit: Tank must store air. This is not for a professional application, and will be "small scale" (in terms of both size and pressure).

• A balloon? A single acting cylinder with a spring load, or gravity load? Or a bellows, perhaps (e.g. inside a rigid outer container pressurized with air or another fluid) Nov 18, 2021 at 1:34
• @PeteW All of these work, but I'm wondering whether I can get one premade or whether I have to custom design one for my application. Your last solution is one I hadn't thought of, however! Nov 18, 2021 at 1:43
• Help me with what you're going for here. A simple hot water tank expansion tank is constant pressure and all tanks are basically variable volume if you're counting the liquid in them. Nov 18, 2021 at 2:54
• There are ones used on hydraulic systems on agricultural machines with nitrogen at about 400Bar - make sure you know what you are doing… Nov 18, 2021 at 5:04
• @TigerGuy Sorry, I should have been more specific. It's for an air tank, which must be able to empty gradually and do work on a gas turbine. To ensure that the turbine has a constant output, it needs to be constant pressure so m(dot) is constant. Since temperature should also be constant, it needs to be variable volume. Nov 19, 2021 at 1:17

Rolling diaphragms are nothing new.

The shaft connected to the rolling diaphragm projects through the housing (unlike the image shows). You generally pressurize the fluid through the shaft. Creating a vacuum on the fluid can cause the membrane to invert; cycling through full stroke can reset the membrane.

The image was captured from https://www.belloframdiaphragm.com/diaphragm-operation

Using something like this, you can have an unpressurized source, pressurize a small volume using this device, and at the end of stroke, draw in more fluid and repressurize. Flapper valves or one way valves allow you to control the direction of flow.

Whether your fluid is compatible with the membrane requires contact with the manufacturer.

Pressurization would be limited by the capability of the membrane.

Just an idea.

Certainly. In the San Francisco bay area there was a huge cylindrical natural gas storage tank which was a huge cylindrical bladder capped off with a conical lid. Gas was pumped into the bladder which caused the lid (which possessed a fixed amount of weight) to rise upwards within a constraining gridwork of girders. the lid rose and fell according to gas demand but the pressure inside the bladder remained the same at all times.

This was near a freeway in Oakland, and if you drove past it on different days, the conical lid stood at different heights!

• Everywhere in the civilized world had gas storage tanks. Some for nat gas , some were water/coal/producer gases. They were essentially gone before I noticed they were being removed. In most of the US, gas transmission pipelines serve the purpose by "packing" - building pressure at night for the next day. Nov 20, 2021 at 17:44