Petrol Stations - Effects of Stage 1B Vapour Recovery on Hydrostatic Guages
Ref / FS- PAN727 / Issue/Revision Date / 21/01/2014 / Review Date / 01/10/2014 / Version: 3.0
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The attached article by Mac Cox published in the February 1997 edition of the APEA's Bulletin is copied for your information.
After reading this article you will be aware of the adverse effects that conversion to a Stage 1B vapour recovery system has on hydrostatic gauges.
With all existing filling stations with a throughput greater than 1000m 3 (1 million litres) per year or irrespective of throughput, filling stations located under permanent living quarters 1 or working areas having to install Stage 1B by 31 December 1998, there will inevitably be a number of those sites using hydrostatic gauges.
Where applications/notifications are received to install a Stage 1B system, the acknowledgment or approval letter should also advise the licensee/operator of the need to replace analogue gauges or modify electronic hydrostatic gauges as appropriate unless it is known that the site uses dipsticks or non-hydrostatic gauges.
At the same time it will also be expedient to draw the licensees/operators attention to Regulations 7 and 9bii of Schedule 12 to the Carriage of Dangerous Goods by Road Regulations 1996 and remind him/her of the need to update the training given to their competent person(s).
1 A number of police stations are known to provide living accommodation on the upper floors and may well fall within this criteria.
RM/CF: 24 June 1997
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THE EFFECTS OF STAGE 1B VAPOUR RECOVERY ON HYDROSTATIC GAUGES
By Mac Cox Technical Services Manager, Veeder Root
FIG 1 : HYDROSTATIC GAUGE, NO STAGE 1B
Introduction
A large number of hydrostatic tank gauging systems are still operational on filling stations in the UK, bothelectronic and an analogue versions of the product. Stage I B has a major impact on the operation of hydrostatic systems which were installed prior to the site being
vapour balanced and the gauges will either need to be modified to cope with the new venting arrangement on the site or, in the case of analogue gauges, replaced, as no upgrade is possible.
The purpose of this paper is to explain the reasons why the modifications or replacements are necessary, what is involved in the modifications and what options are open where replacement is required. Before doing this a brief look at the history of hydrostatic gauges is appropriate.
Hydrostatic Gauges How they Work
The gauges, both analogue and electronic versions, work by measuring the static head of pressure of the liquid in a tank with standardised reference. and then using the resultant reading to infer height which is then converted to volume.
In the analogue version a small hand pump is integral with the analogue dial, and when the system is primed with a few strokes, air is passed, via a restrictor (to aid stability) through the capillary line to the vertical tube in the tank. This air slowly "purges" the product from within the tube until a few bubbles of air emerging from the slot at the bottom end indicates that thc air pressure in the system has created a “balance" with the static head of the product in
The tank, this is the reason why this tube is referred to as the "balance chamber". This pressure balance is seen at the gauge as a repeatable reading between pump strokes. The gauge measurement being made as the pressure expanding the "barometric capsule" against a calibrated, sprung preloaded mechanism causes the pointer to rotate around the scale. The indicating scale is marked up in litres, equivalent to the product height that the measured pressure equates to. The same method if implementation of the hydrostatic
principle is used with the LS1 and LS3 electronic gauge systems. There are a few added sophistications however!
The hand pump is replaced with a small compressor, the dial gauge is replaced with a high precision transducer and micro processor to measure the pressure and convert it to volume. The computed inventory is available on an integral display, but is typically transmitted using an RS232 link to the pas or Pump Controller often as part of a basic wetstock control package.
History of Hydrostatic Gauges in the UK
In the main the systems were supplied by Normond Instruments and are most commonly recognisable under the brand names of the Normond LSI and the Normond G Series analogue gauges. Normond instruments was acquired by VeederRoot in January 1992. but the systems where still sold under the Normond name. The analogue gauges have been in use in both the retail petroleum and industrial markets since their introduction in 1945. In the retail petroleum industry they were widely used as the solution to providing a remote reading tank gauge when remote fills were used, or often simply as a convenient alternative to dipping the tanks. Certainly they were never intended to provide greater precision in stock
measurement and control, as the accuracy is about the same as a dipstick. These gauges are still in production although no longer intended for petrol stations.
The LS 1 gauge, was amongst the first electronic gauges introduced on to the market back in late 1985 and they still comprise a significant percentage of the installed base of electronic gauges in the UK. Although the principle of operation is the same, the electronic versions differ from the analogue one, not only because of the method of pumping the air through the system, but also in that the far more accurate pressure measurement and pressure to height
conversion is made in the electronic version. This meant that the gauge was able to provide enhanced accuracy measurement and control over previous methods. Production of the LS 1 ceased in November 1992. Although better than the G series gauges, the LS 1 systems
did still suffer from some drawbacks, hardly surprising if you consider the extent to which microprocessing technology has advanced over the past twelve years or so since the LS 1 was launched. These limitations are both related to the inherent accuracy of the technology and associated electronics, as well as to the variables apparent when the measurement relies oh specific gravity. But more on this later.
The Effects of Stage 1B on Gauge Operation
Again taking the example of the analogue version, the barometric capsule expansion is due to the pressure in the capsule being greater than the normal atmospheric pressure on the outside of the capsule. Where the tank is normally vented (no stage 1B), the same normal atmospheric pressure is present inside the tank ullage space. As long as the capsule and the ullage space are both at normal atmospheric pressure, the reading on the gauge is representative of the tank contents. When the tank is converted to stage 1B, however, the tank ullage space can no longer be assumed to be at normal atmospheric pressure. Sometimes, it will be at a positive pressure (possibly following a delivery) or at a negative
pressure (possibly during sales). Sometimes the ullage space will be at atmospheric pressure, depending on the type of control valve used in the manifolded vent system, but since this cannot be predicted, this type of gauge is unsuitable for use once stage 1 B is implemented. This restriction applies only to the analogue gauge, not to the LSI or LS3.
Note: It is worth mentioning that the differential between ullage spare pressure and normal atmospheric will result in a level difference between the talk and the fill pipe on direct fills.
If a positive pressure is present in the tank, the level in the fill pipe will be higher; and if negative, the level will be lower: This makes the use of dipsticks difficult!
The LS I and LS3 are able to function in a stage 1B environment as the reference used by the transducer is not the space around the transducer (as it was the space around
the capsule in the dial gauge), but a dedicated reference continuously sampled during every measurement cycle. The micro processor and its application software can handle more than one reference, and allocate them to any of the tanks being measured (up to the maximum of 10 tanks with the LS3). In practice. this enables the manifolded vent system to be the reference for the spirit tanks, while the normally vented derv tanks can continue to be the
referenced to atmosphere.
FIG 2: HYDROSTATIC GAUGE, STAGE 1 B
Modification of Gauges
Where the existing LS1 gauge is being retained for use post implementation of stage 1B, the additional reference for the micro processor and transducer requires a capillary to be run to an access point on the manifolded vent system. This access is typically via a special riser fitted to the existing balance chamber entry on the tank lid, or the point at which the individual vent lines are manifolded if more convenient for ducting back to the LS I position. The LS I
controller will need to be reprogrammed to sample the new spirit reference and to allocate the measured parameter to those tanks. (and only those tanks) that are connected to the
manifolded vent. This is accomplished using custom PC software and an EPROM programmer on site to endure the data matches the site configuration. Planning is vital when considering conversion to stage I B, ensure part of this is checking the type of gauges fitted and their suitability. It is recommended that a qualified contractor is entrusted " with this work to ensure gauge accuracy is maintained. Veeder Root are able to offer complete support for the whole LS product range from informal advice to on site conversion by our national Service Engineering Team.
FIG 3: LS1/3 GAUGE, WITH STAGE 1 B IMPLEMENTED
Upgrading Hydrostatic Gauges - The Options
Where Stage I B has been implemented on a site with analogue gauges, they have to be replaced with another tank gauge. The best option would be to replace it with a modern
system using state of the art technology like magnetostrictive probes which are able to provide highly accurate measurement and stock control, as well as a host of other important benefits, such as the ability to test the tanks for leaks to the required standards without the need or expense of using mobile test. This is especially true in light of the fact that the equipment cost of such a new system is not significantly greater than the alternative detailed below. However it was recognised that there would be significant cost saving to the station owner if the existing balance chambers and nylons could be used for the new systems as this would obviate the need for modifying the tank lid to accommodate a new probe or to lay down new underground ducting for the cables. The need for a cost effective system was even more important when it is considered that the majority of sites which have analogue
gauges are not generally slated for extensive investment or upgrading. With this in mind, VeederRoot decided to launch the LS3, a significantly improved version of the LS
l.
The LS3 operates on the same principles as the LS 1 , however the following improvements have been made:
• Improved pneumatic control ~ displace far less air than the original LS 1 in fact only against the original system!
• Current electronics hardware and software enhance transducer accuracy.
• Inbuilt diagnostic software and “history log" allow service technicians to check on the reason for malfunctions well after they occurred, giving quicker, more reliable fixes.
The LS3 can be installed with minimum on site work no new probes or modifications and minimal new cabling or nylons making it the most cost effective route available for upgrading the analogue gauge.
However, it should still be recognised that hydrostatic systems, no matter how modern. do still suffer from some limitations. primarily that their accuracy varies with changes in specific gravity of the product. This is important particularly as load on load variances of the same grade are not uncommon.
We would therefore recommend that serious consideration is always given to upgrading to a current technology as this may well be more cost effective in the long term.
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