Flexible Connectors

One of the most important design considerations of any chemical storage tank system is the use of flexible connectors and isolation valves. When paired correctly, these accessories will not only help ensure your storage tank’s longevity, they can also protect the tank from hinge points that cause dangerous leaks.

Photo of a flexible connector and isolation valve properly installed on a plastic chemical storage tank

Let’s take a closer look at flexible connectors and isolation valves—and at a danger that may be hiding in plain sight.

Why Use Flexible Connectors?

One of the biggest advantages of polyethylene chemical storage tanks is that they expand and contract with pressure changes during loading and unloading and during changing weather conditions. While a rigid tank is more susceptible to cracking and damage over time, a flexible tank is resilient to pressure.

However, a polyethylene storage tank is only as good as the fittings and accessories you pair it with. For many years, the industry standard was hard piping that came directly into the side of the tank. Because hard piping won’t move with an expanding or contracting sidewall, it can damage a polyethylene tank in several ways:

  • Piping vibration from pumps
  • Fulcrums that crack the tank
  • Hinge points that stress the tank

Flexible expansion joints allow the tank to move as it should while avoiding the damage that rigid piping can cause. However, you’ll also need an isolation valve as a backup feature to prevent loss of product.

Selecting Flexible Expansion Joints for Your Tank

Photo of a flexible expansion joint and isolation valveAs you design your tank, you’ll need to consider the location of the flexible connectors. The bottom third of the tank sidewall is the most critical area for flexible connectors, because that is where the most movement occurs in a polyethylene tank.

It’s also important to identify the best type of expansion joints for your application. Some large linear tank manufacturers tried using common expansion joints on their tanks, but the tanks continued to break at the fitting area. It’s important to use the right type of expansion joint and the right type of tank.

Poly Processing offers the PTFE Expansion Joint.

Diagram of how the PTFE Expansion Joint works

PTFE Expansion Joint

The PTFE Expansion Joint is a PTFE in-line flexible connector that accommodates the tank’s expansion and contraction, absorbs damaging shock and vibration, and compensates for misalignment in the piping it feeds.

Flexible Connection Minimum Specifications are below: (For common size fittings and piping systems up to 6”) Contact Poly Processing for information on larger expansion joints.

  • Axial Compression ≥ 0.67”
  • Axial Extension ≥ 0.67”
  • Lateral Deflection ≥ 0.51”
  • Angular Deflection ≥ 14°
  • Torsional Rotation ≥ 4°

Before Installing Your Flexible Connections

The fittings and accessories are a very important component of designing your chemical storage tank system. Before selecting a flexible connection, there are a few things to consider.

First, you need to understand the space requirements of your tank system. What will the total dimension be of the plumbing with the expansion joint and the tank system?

It’s also important for your flexible connection to be supported properly, but the support shouldn’t restrict the horizontal plane movement of the tank.  When using an expansion joint, the pipe support should be placed after the flex to allow the tank to move outward when filled.

First photo example of flexible connection properly supported

Second photo example of flexible connection not properly supported





If you’re using flexible connectors, it is critical to specify them in the correct specifications. Sometimes engineers will specify flexible connectors in the plumbing specs, but we only receive tank specs. We recommend including any flexible connectors that you require in your tank specs as well, so we can see them, do a review on the specs, and design a proper tank system.

Once you’ve selected your flexible connection, install it in the correct spot. In most cases, flexible connectors only need to be installed on the lower third of the tank sidewall, since this is the section of the tank that moves the most. Expansion joints should not be attached directly to the tank wall or IMFO. A full face flange must be installed on the tank first. We recommend installing an isolation valve between the tank and expansion joint. This isolation valve lets you shut off the fitting in case of a failure at the flexible connector or other part of the plumbing system.

Introduction to Isolation Valves

An isolation valve is a valve that stops the flow of chemicals in or out of the tank. It is installed for maintenance or safety purposes, and you should have an isolation valve at key intersections along with the flow of travel. If a leak develops in the piping or fittings, the valve can be closed upstream to prevent further loss. This saves on the chemical costs and the hassle of cleanup and downtime.

An isolation valve is operated by a quarter turn and uses a floating valve for closure. The position of the handle tells you at a glance if the line is open or shut, giving you peace of mind that the valve is sealed. There are several types of isolation valves, including ball valves, single body isolation valves, top entry isolation valves and split body isolation valves.

Isolation valves are also relatively inexpensive. One-piece isolation valves are easy to replace, two-piece versions are easy to clean, and three-piece versions are easy to repair.

A Danger Hiding in Plain Sight

Improperly installed valves and expansion joints can put your chemical tank at risk. If a leak occurs at the joint, you won’t be able to shut off the flow and you will lose good product.

Our field service teams have seen several examples of improper installations at customer sites. In each case, the facility was confident that they were protected and safe. However, their configuration put them in danger of unnecessarily losing a great volume of product.

For a flexible connector to serve its purpose, it has to be installed correctly and supported properly. The problem arises when some customers place the expansion joint between the fitting and the isolation valve. The purpose of the isolation valve is to shut off the flow if the expansion joint starts to leak, but when the valve is placed after the expansion joint, there is no way to shut off the flow. In this case, a failure in the expansion joint will be unstoppable and the entire contents of the chemical tank can be lost—simply due to improper installation.

You can rely on Poly Processing to ensure that your flexible connectors are properly installed and functioning as intended.

Please refer to pages 26-29 in our IOM Manual for detailed information regarding flexible connectors. Click here to go to the IOM Manual.

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