Passing Gas (Part 1)

Continuing our discussion from last week, we're talking about dosing chemicals that off gas.  There are pumps with degassing heads, pumps with internal sensors that initiate a degassing sequence when they deem it necessary, some pumps utilize timers, and some pumps simply pass gas.  The nature of the design of a peristaltic tube or hose pump lends itself well to just passing the gas.  We're gonna talk pros and cons of that design this week.  I have borrowed a page from a pump manufacturer's site that explains some of the design features of a peristaltic pump.

"Peristaltic pumps provide excellent problem solving pumping solutions especially when the product being pumped is particularly abrasive, corrosive or viscous. Their lack of valves, seals and glands makes them inexpensive to maintain the only maintenance item is the hose or tube. Peristaltic pumps also have a gentle pumping action, ideal for shear sensitive polymers and fragile cell cultures. Lastly, the only part of the pump in contact with the fluid being pumped is the interior of the tube or hose, making it easy to sterilize and clean the inside surfaces of the pump.

Seal-less design

Peristaltic pumps have no seals avoiding issues such as leaks of corrosive chemicals and ongoing maintenance.

Low maintenance costs

The only replacement part is the hose or tube a relatively low cost item that can be easily changed in a short time.

Dry running and self-priming

Peristaltic pumps do not require pumped fluid to be continually present pumps can run dry, without costly downtime or repairs. The recovery of the hose or tube creates a powerful self-priming action and allows the pumps to move liquids containing entrapped air or that can off-gas.

Gentle pumping action

Peristaltic pumps have a gentle, low shear pumping action, ideal for shear sensitive products including flocculants and broths

High suction lift

The powerful suction provided by the recovery of a re-enforced hose gives hose pumps suction lift capabilities up to 9.5m or 31 ft of water.

Abrasion resistant

Hose life is not related to a product’s abrasive qualities. The hose only fails due to fatigue or chemical action.

Solids handling

Hose pumps can pump slurries containing up to 80% inorganic solids or 15% organic sludge.

Reversible

Peristaltic pumps are reversible and can be used to empty lines or clear blockages

No slip

The pumps have no internal backflow giving accurate dosing without slip

Accurate dosing

The pumps are accurate in dosing; they have a repeatability of ± 1% and metering capabilities of ± 5%."

What the manufacture doesn't discuss:

Peristaltic  pumps are designed to fail.  The act of compressing the hose over and over again, causes hysteresis fatigue and eventual failure.  Unfortunately, it's a guessing game as to when the failure will exactly occur.  Knowing you have a pump that fails, ideally you would like to change hoses just prior to failure and at your convenience.  But as most of us know, it tends to happen over the weekend, when manpower is minimal, and if you don't buy all the bells and whistles that are available to alert you of your failed hose, you may have learned only after draining the tank of your chemical.  While you can't predict exactly when your tube/hose will fail, once it does, provided your application really doesn't change much, you have a pretty solid benchmark timeline of when you should schedule your maintenance.  I have outfitted some pumps with counters to count revolutions to account for the number of times the hose has been compressed and assisted with creating maintenance schedules based on that.  

Peristaltic pumps are limited in the available hose and tube options and the compatibility with your chemical.  While some tubes offer great chemical resistance, they are limited in your flow rates.  And the larger the pump, the fewer and fewer options you have for material choices.  

Dosing volume will change over time.  Due to wear of the tube or hose and the hysteresis it will experience, there will be a change in volume pumped per revolution.  If dosing accuracy is a concern, this will require a VFD to be able to adjust the speed to maintain consistent flows over time.

I have heard sales people suggest that pulsation dampeners are not required with peristaltic pumps.  That really depends on your system and what size pump we're talking about.  Peristaltic pumps are not free from pulsation.  They can pulsate as bad, if not worse, than many AODD pumps or diaphragm style metering pumps.  To achieve less pulsation, you have to run the pump much slower, which means you may be over sizing your pump to achieve the reduced pulsation.  Once again, depending on your system, you could be spending more for the larger pump, than you would for the smaller pump and pulsation dampener.

Peristaltic pumps have pressure and temperature limitations as well.  The max pressure any hose pump can do is limited to 232 psi or 16 bar.  And the hose is limited to temps up to 180*-210* depending on what material is selected.    While 232 psi is the max any hose pump can accomplish, many max out in the 60 to 116 psi range.  You may think that is not really a concern when pumping sodium hypo, the example we have used from the start while discussing liquids that gas-off, you'd be surprised as more and more municipalities are having to dose at their booster stations in order to meet residual requirements in the lines furthest from their treatment facilities.  And some times, a requirement like that, dictates the need to look at a diaphragm style metering pump.  We'll discuss more about that next week.  

Peristaltic pumps can be an excellent choice for you when handling liquids that off-gas, just be sure you do your due diligence to determine whether it's right for you!  If you are contemplating whether it's the right choice for you, feel free to call me @ 815-412-5683 to help review your application, look over your system, and determine what available options you may have.

Steve Shapiro

Dosing chemicals that off gas

The following is an article that describes the difficulties often experienced when dosing chemicals that off gas, specifically Sodium Hypochlorite. This week we identify some challenges and some of the more traditional ways these problems are corrected. Next week we will review newer methods and technologies that are used to overcome this problematic application and other applications like it.


Technical Guidelines for Sodium Hypochlorite Metering

Introduction

Often, Sodium Hypochlorite can be difficult to successfully meter into a process for disinfecting purposes due to its chemical composition and tendency to “gas off” during hot ambient temperatures in particular, the problem being more prevalent in warmer climates…………..Why?

What is a Sodium Hypochlorite solution?

Sodium Hypochlorite solution consists of the elements Sodium Hypochlorite (NaClO) 10%, Sodium Hydroxide (NaOH,Caustic Soda) 1%, Sodium Chloride (NaCI) 9%, and water. It is an oxidizing agent

Why do Sodium Hypochlorite solutions “Gas off”?

Sodium Hypochlorite solutions “Gas off” because the Sodium Hypochlorite de-composes in your tank and pipes over time into a salt solution and generates Oxygen in the form of bubbles, the bubbles accumulate in pump suction lines causing loss of prime.

The de-composition rate is a factor of Hypo concentration and ambient conditions.

The higher the concentration and ambient temperature, the faster it will “Gas off”.

Hypo will de-compose down to a 5% solution then stop!

1ml of active chlorine in solution will produce a quarter gallon of gas!

How do we minimize “ off-gassing” problems with our metering pump installations?

Keep the Hypo Solution cool by installing your tank out of direct sunlight and away from heat sources the best you can.

Consider insulation of the tank, metering pump suction lines or both.

Cool hypo will dramatically reduce metering difficulties

Reduce the concentration if it’s an option.

Install the largest practical pump possible to allow better volumetric efficiency. This will allow accumulated bubbles to be forced through the pump head without the need for complex gas bleeding devices.

How do we install our metering pumps correctly?

Suction pipe work should be inclined downward to the pump slightly to allow oxygen bubbles to evacuate back into the tank and vent to atmosphere.

A vertical “stand pipe” (just before the pump inlet), rising above the level of the tank and protected from foreign ingress can help prevent gas accumulation. Any gas which builds up in the suction pipe is automatically evacuated to atmosphere before reaching the pump inlet. Alternatively, a suitable priming aid can be installed.

Manual or automatic air vent valves should be fitted to the discharge pipe work to aid in priming. Remember: gas bubbles cannot be forced through a spring-loaded dosing or back pressure valve.

A water flushing circuit “Teed” into the suction line will aid in flushing out any salt residue in suction lines and pumps.

Treat each Hypo installation individually - location, temperature, piping, levels, dosing rate, discharge pressure all need to be considered case by case.

Summary

• Sodium Hypochlorite solutions naturally decompose and form oxygen bubbles and some salt residue in the tanks and piping causing “Gas off”.
• The higher the ambient temperature - the more difficult Hypo will be to dose.
• The higher the Hypo concentration - the more difficult it will be to dose.
• The smaller the dosing rate - the more difficult Hypo will be to dose.
• The smaller the metering pump - the more difficult Hypo will be to dose.

Controlling Fluid Dynamics

Written by Gary L. Cornell, BLACOH Fluid Control, Inc.   

Pulsation and water hammer can be limited with proper forethought and equipment.

The control of fluid dynamics is essential to ensure efficient, reliable and safe operation of pumping systems. A pump puts fluid in motion by adding energy to it. This kinetic energy, observed as pressure, is carried in the fluid and slowly lost to friction in the piping system. Uncontrolled fluid in motion can physically destroy the pump, piping, valves, meters and other system components. 

Positive Displacement Pumps

Positive displacement pumps rapidly accelerate and decelerate fluids that are in motion. They derive their pumping action by capturing a specific quantity of process fluid in a chamber and then pushing it out of the pump’s discharge. During the pump’s suction stroke an inlet valve is raised and an outlet valve is closed, allowing fluid to enter the pumping chamber. On the discharge stroke, the inlet valve is forced closed. Hydraulic pressure created by the pump’s piston opens the outlet valve to push the fluid out the discharge. This start and stop pumping action accelerates and decelerates the fluid creating units of uncontrolled kinetic energy resulting in pulsations observed as pressure spikes. Vibration is the most visible effect of pulsation and the problem that most often leads to system component failure.

        Single diaphragm metering pumps create a start and stop action resulting in wide pressure fluctuations. With each stroke of the pump a small volume of fluid is discharged that must re-accelerate the fluid in the piping. The pump then has to overcome the resulting spike in pressure to continue to discharge process fluid.

        A peristaltic pump, also called a hose pump, has a hose inside the case. A roller shoe at the pump inlet squeezes the hose trapping liquid in the tube ahead of it. As the roller shoe rotates, liquid is pushed out the pump’s discharge. When the roller shoe releases the hose after discharge, a momentary void is created, and a partial vacuum results as some product is sucked backed to the discharge. This action, along with the normal pulsations from the pump’s positive displacement nature, makes dampening the discharge flow on a peristaltic pump difficult.

        The same pulsing action and pressure variations occur at the pump’s inlet. As a roller shoe passes across the pump inlet and closes it off, flow into the pump momentarily stops. If the pump inlet is under positive pressure, acceleration head will cause damaging pressure spikes and vibration. If the pump inlet is under vacuum, cavitation and pump starvation can occur.

Pulsation Control

Options for minimizing pulsation damage include using heavy walled pipe; additional pipe braces; snubbing devices on equipment; and sometimes, back pressure valves. Generally, dampeners provide the most compact, efficient and cost-effective method available to control pulsation. The most common type of dampener is a hydro-pneumatic pressure vessel containing compressed air or nitrogen and a bladder or bellows that separates the process fluid from the gas charge. The dampener is installed as close as possible to the pump discharge with a gas charge that is slightly below normal system pressure.

        The amount of pulsation absorbed is a function of the dampener size to pump stroke volume. The pulsation dampener absorbs the pressure spikes created by the rapid acceleration of fluid from the pump’s discharge. On each pump stroke, the dampener fills with process fluid and then discharges some of the accumulated fluid when the pump is on its suction stroke to keep the fluid in motion. By controlling the fluid in motion, the dampener prevents system piping fatigue, enhances meter performance and protects gauges and other inline instrumentation. By minimizing pulsation, dampeners can be particularly beneficial in filling, spraying and chemical injection applications where an even and continuous flow is required.

Water Hammer/Hydraulic Shock

While pulsation is the effect of rapid acceleration and deceleration of fluid in motion, water hammer occurs when fluid in motion is suddenly started, stopped or forced to change direction. Whenever fluid velocity changes rapidly, water hammer should be anticipated.

        Fluid velocity, volume and density all contribute to the pressure spike created when fluid in motion suddenly stops and kinetic energy is released. The kinetic energy, released as pressure, can spike up to six times the system’s operating pressure, destroying system instrumentation, pumps, pipes, fittings and valves. Unrestricted, this high-pressure surge, commonly referred to as hydraulic shock or water hammer, will rapidly accelerate to the speed of sound in liquid creating an acoustic wave or transient which can exceed 4,000 feet per second. The water hammer shock wave travels the length of the pipe back to the pump, and then reverses again. It oscillates back and forth until friction dissipates the pressure spike or the weakest component in the system fails. Water hammer should be suspect whenever fluid velocity is 5 feet per second or greater.

        Quick closing valves, rapid pump startup/shutdown and even changes in the pipe profile can cause an abrupt change in fluid velocity, which can produce violent and sometimes catastrophic water hammer. For example, if the pipe is full of liquid at pump startup, that stationary liquid must be accelerated. When the pump pushes liquid into the pipe it hits the stationery liquid. When a pump shuts down, liquid continues to move down the pipe due to momentum resulting in a void at the pump discharge that is an area of low pressure. The liquid in the pipe can reverse direction into this area striking a check valve or the pump itself. A momentary power failure can create even greater hydraulic shock. When the pump stops, flow will reverse back to the pump just as power is restored and flow is restarted causing a head-on collision between the two water columns.

        Generally, the most common cause of hydraulic shock is a quickly closing valve usually defined as a valve closing in 1½ seconds or less—typically a ball or butterfly type. Flow velocity is stopped rapidly, energy is concentrated and an acoustic shock wave is created. Air relief valves, vacuum breaker valves and pressure relief valves are often used in specific areas to help mitigate hydraulic shock resulting from fast closing valves.

        One commonly used solution for controlling hydraulic shock is a surge suppressor. This device is similar in construction to a pulsation dampener but sized and installed differently. The surge suppressor acts as a reservoir or accumulator to absorb and release fluid as needed. By doing so, it controls the rate of velocity change to a level slow enough to prevent water hammer. Three guidelines must be followed when using surge suppressors to prevent or minimize water hammer:

• The device must be located in the correct area.
• It must be sized properly to accumulate the correct amount of liquid.
• It must be pre-charged with nitrogen to provide the proper shock control.

Every fluid system is different and many can be complicated. This article is designed only as an introduction, and in many situations, professionals in the field of hydraulic shock should be consulted, especially before construction of a new system.

Gary Cornell is Chairman/CEO of BLACOH Fluid Control, Inc., a manufacturer of pulsation dampeners, surge suppressors, inlet stabilizers and other fluid control products and based in Riverside, Calif. With a BS degree from California Polytechnic University, Cornell has worked in the reciprocating pump industry for more than 35 years and is a member of the Hydraulic Institute and the American Society of Mechanical Engineers.  The Panner Company is a master distributor of Blacoh products and has the ability to help you size and supply you with the equipment needed to eliminate water hammer and surge events in your system.  Call them at 815-469-8333.