Fluid density effect on pipe plug

Inflatable Pipe Stoppers are made off reinforced rubber and block the flow to seal the pipe lines. After plugging/sealing of the pipe line, liquid will rise at back side of the plug. That will create Back Pressure.

What is the Back Pressure? Back pressure is the level difference between the upstream and the downstream ends of the pipe line; or, in other terms, the height of the liquid (water, oil, petroleum etc.) column behind the plug. That liquid column pushes the plugs from back side and force it to move. The kind of liquid is important. When we review the below formulas, we will understand it clearly.


The pressure is directly proportional to weight. If the weight on the unit square area increases, the pressure increases on the plug. Let’s compare the two-type liquid such as water and petroleum for the case of sealing 1000mm vertical pipe.

Case: The density of water is 1000 kg/(m^3) and density of petroleum is approx. 800 kg/(m^3)


  • S: cross-sectional area
  • F: Force
  • P: Pressure
  • g= gravitational acceleration N/kg/
  • V= volume


We assume the volume is fixed and equal to 1 m^3,g=10 N/kg

  • The weight of water will be 10000 kg.
  • The weight of petroleum will be 8000 kg.

Now, we that cross-sectional area is 0,7854 m^2, for 1000mm(1m) diameter,

So, the pressure will be


As we can see, the pressure back side of plug depends the liquid’s density.

When liquid column pushes the plug, the plug resists that force by friction force. The friction force can change according to pipe material. Generally, the friction force is low for the HDP and GRP pipes. However, it is high in the steel pipe and concrete pipe.


The pipe plug manufacturer generally calculates the back pressure for dry concrete pipe when they inform to client about back pressure. Back pressure depends a few variable things such as pipe media, liquid type, pipe diameter, the height, and inflation pressure. Due to that variables the manufacturer cannot inform the back pressure for each application. Because each application has different friction force. You can see the notes in the manufacturer catalog, the back pressure calculated for what type of pipe. PlugCo calculated the back pressure for dry concrete pipe.

So, the end users should know that above facts while they selecting the inflatable pipe plugs and follow the safety rules during application.