The alternative method to increase the usage life of gas cylinder is to reduce the product losses and improve the consumption characteristics. To consider this, a model for the predictive life of a gas cylinder was built by modelling consumption, see Equation 2. In considering the gas losses (MSV) two main areas were identified to model, the surge of gas as a system starts to flow and the stability of the flow rate during the use of the cylinder. Whenever a pressurised gas system transitions from a static condition to a flowing condition, a surge of gas is experienced as the gas flows into the low pressure regions (Pemberton G. , 2014a). In such a scenario the gas regulator opens in order to meet the flow demand and in doing so it will over-compensate (as no previous flow existed). The result is a surge of gas. This phenomenon is not well published, although it is identified in the field of metals fabrication as a source of gas loses. The term ‘weld surge’ is used to describe lost gas due to triggering the gas flow. A calculation model for the gas surge loss was built, Equation 3.
𝑈𝑠𝑒𝑎𝑔𝑒 𝑡𝑖𝑚𝑒 (𝑚𝑖𝑛𝑠) =(𝑀 − (𝑀𝑆𝑉∗ 𝑛)) (𝑓𝐴𝑉𝐺)
Where M – Initial mass of gas (for a full cylinder) in Kg; MSV – Gas loses due to surge in Kg; n – Number of gas openings; fAVG – Average flow rate (Kg/min)
Equation 2: Model of gas consumption
𝑉𝐺𝑆= ∫ 𝑓𝑑𝑡 − (𝑓𝑆𝐸𝑇∗ 𝑡)
𝑡
0
Where VGS – Volume of gas due to surge (l); f – instantaneous flow rate (l/min); fSET – flow rate set for application (l/min); t = total application time (mins)
Equation 3: Quantification of excess gas
Considering the stability of flow at the same time as the surge effect builds a predictive model for gas cylinder life, Equation 4. The model is then used to review design parameters by simulating real life gas flow scenarios when welding. By varying the model’s parameters, it is found that a gas surge volume between 0.2 and 0.4 litres is optimal, see Table 3 and Figure 18. Less than 0.2 litres brought little additional user savings but increased the technical difficulty and product costs. In terms of stability a target of +/- 15% was found to give the most user benefit without making the system difficult to technically achieve, see Table 3 (Pemberton G. , 2014a).
𝑥 = 𝑥𝑖− [(𝑓𝑖𝑇𝑖) + (𝑆𝑖𝑇𝑖)]
Where x – current cylinder content; xi – initial cylinder content; F – flowrate; T = time step; S = surge loss; f = fi.f(xi) [flowrate is a function of cylinder content]; S = Si.f(xi) [surge volume is a function of cylinder content]
Equation 4: Predictive model of cylinder life
Case Duration
No.
Openings Gas on time Gas
consumed by surge
Gas consumed by flow
Hrs Litres Litres
Base case Short 6000 5.00 9000 5140
Base case Std 1980.0 11.0 2970 11320
Base case Long 750.0 12.5 1125 12840
Perfection Short 18600.0 15.5 0.0 13950.0
No Surge Std 2760.0 15.3 0.0 13800.0
No surge Long 930.0 16.8 0.0 13950.0
No flow variation Std 2220.0 12.3 3330 11100
No flow variation Long 870.0 14.5 1305.0 13050.0
Table 3: Table of results for cylinder life calculation by performance variation
Figure 18: Graph of surge volume against usage time for short cycles
Research was then conducted into devices which reduce surge and/or improve stability. So called gas economisers’ are available in the market and were found mostly in the metals fabrication area.
Although some exist in medical Oxygen delivery systems (Barker, Burgher, & Plummer, 1994). The current product base has impressive claims about potential savings of up to 50% (Fit Up Gear, 2014).
However the actual cost impact maybe limited as gas costs are a small contribution to the overall production costs, typically, between 1% and 6% (Cambell, Galloway, Ramsey, & McPherson, 2012).
The economisers found can be categorised into four technical solutions; flow regulators, fixed orifices (flow limiters), pressure regulators and quick acting valves. Each has their specific benefits and drawbacks, for instance, fixed orifices perform well at restricting the maximum flow and are low cost, but the gas loss due to surge is not well addressed. Flow regulators are an improved product, exhibiting good stability but are expensive compared to other options. Therefore their overall monetary saving can be questionable. While quick acting valves (i.e. solenoid valves) do achieve minimal pressure build up and good flow control, these systems are often more than ten times the price of other solutions. A gap was identified for a product, with an ability to control the pressure build up and manage flow at reasonable costs. An ideal device would have similar performance to quick acting valves and be of comparable costs to other devices.
The other main contributor to gas losses, is flowrate variation. As a gas cylinder is used, the outlet flow will vary as the pressure reduces in the cylinder; typical regulation equipment do not compensate for this variation. There were no directly marketed ‘gas saving’ products found in this aspect.
However, it could be conceived that the use of a two stage regulator, or pressure regulator in 0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
5.00 7.00 9.00 11.00 13.00 15.00
Surge Volume (litres)
Time (Hrs)
Graph of Surge Volume to Cylinder Life for Short Cycles
conjunction with a flow regulator is an available solution to minimise losses. These methods are often applied when a high stability in flowrate is required sych as TIG welding. It is recognised that the most common method to address losses through flow stability is simply to adjust the flow as the cylinder is being used.
Considering this view on product losses, it identifies an opportunity to address issues of surge and flow variation at the same time.
The requirements for a product were then built and a list of parameters created:
Steady pressure delivery
Steady flow rate delivery
A useable pressure range for the application
A useable range of flow rates for the application
Quick closing of the system when gas demand is not required
Control of the initial pressure release to limit the initial flow and manage the gas surge
Simple to use
Low cost
An idea generation event was held to create a starting point for a product development. Eight ideas were chosen to have enough technical and commercial benefit for further work. This led to the selection two position valve system which can be adjusted depending on whether the use was for long or short duration welds. During the ideas session the concept was explored of optimising the valve for different welding conditions. Two main areas of welding were identified tack weld, short in duration and being most sensitive to surge, then penetration welds, long duration and more sensitive to stability. The two position system provides an opportunity for an optimal solution, minimum surge for tack welds (short duration) and improved flow stability for penetration welding (longer duration).
Once developed and tested the system was compared to other offerings, including with gas economisers. Significant performance benefits were seen with the new system. From a base case the cylinder life (time used) could be extended between 11% and 142% depending on the user profile, see Figure 19. Compared to the best in class set up of an economiser, the developed system out performs this by a significant margin (an economiser saves between 5% and 50%). Additionally as the new system was integrated with a cylinder valve it provides a lower cost solution than using an add on economiser.
Figure 19: Chart of gas savings for three usage profiles
The comparison tests of the new system show several benefits from a sustainability view. The first being that the cylinder will last on a customer site between, 11% and 142% longer in time used. Using a generalised approximation of a customer consumption pattern, 1 cylinder per 2 weeks, then over a year the number of deliveries may be reduced from 24 to between 10 and 21. Similarly the number of gas cylinder change overs would reduce by the same amount. The number of cylinders required in the fleet can be reduced by 1 to 2 depending on the processing time of the cylinder plant.
Testing of the product produced positive customer feedback, many commented on the usefulness of the two positions for welding and that the cylinder lasts longer. However, as the time came to launch the product the business managers took a decision to postpone a full launch and take some further time to improve stability in the tack welding mode. The business managers also decided they wanted to take more time to develop the business case and sales approach. Lastly and also very importantly funding was not made available for capital purchases required to further the project due to other priorities. Capital expenditure was limited due to overarching tough business conditions and focused on essential replacement of assets. Funding for all new products was essentially put on hold.