Liquid Pressure Dangers... aka pressure surge (part 1) - 8 minute read

by the way, a google search of "liquid dangers" predominantly refers to vaping / e-cigarettes / juul none of which are referred to in this article. I haven't seen any reports of pressure surges occurring on those devices...

This writeup is the product of an itch…, an itch to determine a reasonable sequence of performing a surge analysis, hopefully, readers would likely educate me further on this vast subject.

For the purposes of this writeup, I’ll refer to a closed cylindrical conduit (pipeline) and not discuss anything open channel related.?

This is what a pressure surge can lead to…

Reference: Sizing the Protection Devices to Control Water Hammer Damage (2013)

So, what is a pressure surge analysis?

A?pressure surge is basically a transient state where the ratio of the potential energy to kinetic energy is changing and delta energy is converted to an acoustic wave.

The?analysis involves assessing the impact of a pressure wave generated due to the change in velocity of a column of liquid. The column of liquid may either have been at rest (velocity = 0) or may have been in motion. The kinetic or potential energy of the liquid is converted to a wave that initially moves away from the source of disturbance at a velocity that is proportional to the size of disturbance.?

Let’s get a bit?academic….

Pressure Transient Process

Reviewing a pressure surge process after instantaneous valve closure…

Assume we have a source or in this case, a water tank with a certain head, supplying water to a pipe with a valve further downstream the pipe. The valve is capable of being shut instantaneously (i.e. at a time much quicker than the pipe period). Also, assume water is flowing in layers down the pipe.

On instantaneous valve closure (upstream valve):?

• Liquid velocity immediately on the upstream side of the valve is reduced to zero. The kinetic energy of this portion of the moving column is transformed into potential energy, i.e., the static pressure increases instantaneously. Velocity at the inlet is not zero (kinetic energy);
• Then there is a marginal decrease in liquid volume due to its compressibility followed by an increase in pipe volume (dependent on the elasticity of the pipe material and the dimensions of the pipe). A pressure wave is set-up in the reverse direction (heading to the inlet, in this case, the tank), at the head of this wave the pressure will be higher than the original pressure at each point it passes on its way to the inlet;?

On instantaneous valve closure (downstream valve):

• Liquid keeps flowing away from the downstream face of the valve, but as there is no replacement of liquid layers, there is a drop in pressure.

Upstream valve:

• The compression of the liquid adjacent the valve allows the rest of the column to continue momentarily to flow normally; but as liquid moves in to occupy the space created by the compression, more of the column is brought to rest with the accompanying increase in pressure. The pipe will tend to expand (dependent on the pipes modulus);?
• Pressure at the upstream face of the valve keeps rising as the pressure wave progresses upstream the pipe, though not as fast as during instantaneous valve closure.?
• Thus the “surge” progresses toward the tank until the entire column is at rest and at an increased pressure. The pressure wave reaches the tank at period L/a, at this point no more water is flowing into the pipe and the liquid in the line is fully compressed.?

Downstream valve:

If the pressure drop experienced is low enough (and dependent on elevation at that point), this could lead to boiling of the liquid. Voids can grow and collapse giving rise to pressure surge.

• The pressure wave then reverses back towards the shut valve after period L/a. The compressed and pressurised layers of water expand and start flowing upstream (back into the tank) in the reverse direction. Maximum pressure reached when the compressed water layer at the upstream valve face flows away from the valve at period 2L/a.

• With the last layer of water flowing away from upstream the valve face, this creates an area of low pressure and a dramatic pressure drop. Creating a pressure wave that travels from upstream the valve to the tank (with falling pressure upstream the valve), stopping the reverse flow of liquid into the tank by the time it reaches the tank at period 3L/a.

The wave reverses back towards the valve on reaching the pipe inlet, and water starts flowing back into the pipe towards the valve.

This cycle is repeated until the kinetic energy of the moving column dissipates via friction and modulation by the pipe wall and surroundings leading to it coming to rest.

Prepping for a Surge Analysis

Sources of Surge Pressure

A typical source of a pressure surge emanates from the opening / closing of a valve (mentioned above) or the start / stop of a pump but could also be triggered by the vibration of the pump at the natural frequency of the pipeline (this is more of a resonance issue typically considered under mechanical design not surge analysis). Resonance refers to oscillations, triggered from pump / valve operations around the harmonic or fundamental frequency of the system and increase in amplitude (without actually exceeding the MIP), but could lead to damage to supports or seals/joints. Resonance is more of an issue in unburied pipe sections and impacts on pipe routing / design of supports.

Pressure surge may be a positive or negative (vacuum) value. The following includes most sources of pressure surges within a pipeline system.

1. Pump start-up – Pump start-up (especially in non-VSD’s or low inertia pumps) may lead to a rapid collapse of the void space downstream from a starting pump, in the process, generating high pressures;
2. Pump shutdown or trip;
3. Pump power failure – Pump power failure could precipitate a pressure upsurge on the suction side and corresponding pressure down-surge on the discharge side. In the down-surge, the minimum pressure could drop to the liquids vapour pressure, resulting in vapour column separation;
4. Pump malfunction (like impeller or turbine vibration);
5. Variations in product demand or reservoir levels or incorrect pipeline filling operation – Changes in demand or reservoir levels or incorrect pipeline filling operations – during pipe filling, faulty controls may allow excessive air to be pumped into the system which may lead to formation of air pockets. These air pockets can accentuate instabilities within the pipeline especially if drained quickly;
6. Valve opening – A valve fully opening within the wave reflection time of 2L/a will typically trigger an excessive pressure surge. A pressure surge will still be triggered even if the valve fully opens within a longer period than the wave reflection time but will be much smaller;
7. Valve closing – Closing a valve within less time than the wave reflection time results in a pressure surge. Closing a valve within greater time than the wave reflection time results in a much smaller pressure surge;
8. Unplanned disconnection of a pipe (e.g. loading arm) – On disconnection of a loading arm, excessive surge pressures may be experienced within the ship's breakaway couplings;
9. Accidental pipeline rupture.

System Components

Typically, a thorough surge analysis for a liquid system (either a single pipe or network of pipelines) will entail an evaluation of the entire system from the liquid tank / reservoir at the exporting facility to the storage tank at the receiving facility. This process could become very, very detailed dependent on the stage of analysis, the selection of the tool for analysis, understanding the simplifications required and so on. The model itself will likely include the following components:

• Pumps;
• Check valves;
• Control valves;
• Piping / pipeline elevation profiles (especially detailed in-plant and around pumps);
• Fluid properties;
• Ambient conditions;
• Pipeline properties;
• Surge mitigation equipment (existing / or suggested after initial analysis);
• Pipeline branches / jumpovers / crossovers;
• Pressure reducers;
• Operation strategies

Satisfying Regulatory Requirements

It is important to fully understand the requirements of both the client (as well as national) standards and also to be able to compare these with relevant international standards. Requirements for some standards are displayed below:

The drawing below further illustrates the definition of the MIP as defined by DNVGL-ST-F101.

Reference: DNVGL-ST-F101, Table 1.8 – Oct. 2017

It is also worthwhile paying attention to any liquid velocity limits designed to limit the impact of pressure surges (e.g. limit below 4m/s) and if there are preferred surge devices for specific systems (e.g. bladder valves), where relevant. Certain surge mitigation equipment and/or measures will protect against specific surge events (such as column separation, line pack, resonance, surge pressures) whilst others will provide protection for a number of events.?

In further articles I'll go deeper into the background pressure surge calculation methodologies, software, pros and cons etc. Comments are welcome!