What are Bellow sealed Valves?
WHAT ARE BELLOW SEALED VALVES?
Bellow Sealed Valves are a new breed of industrial process valves, designed to eliminate Valve leakage and are also known as ‘Zero Leak Valves’ or ‘Emission Free Valves.’
In these Valves, conventional gland packing is appended/ supplemented by a metallic Bellow Cartridge joined to the valve bonnet at one end, and the stem at the other end. A bellow is a long accordion-like tube. As the valve stem strokes, the bellow expands or compresses with the stroke movement. There are no sliding or rotating seals through which processes fluid can pass.
Thus the fluid will enter the valve from the seat of the valve & come in contact with the bellow, but will not come in contact with the stem. Since the stem is the only moving part in the valve. It is a source of potential leakage through the gland due to constant friction caused by the axial movement.
In a bellow sealed valve this potential leak is totally avoided due to the metallic bellow which blocks the leak path effectively by not allowing the media to come in contact with the stem and thus the gland.
A bellow is the heart of this type of valve. There are two types of bellows : formed and welded leaf. A formed bellow is made by rolling a flat metal sheet into a tube with longitudinal fusion welding. This tube is then mechanically or hydraulically formed into bellows with rounded and widely spaced folds.
In Welded Leaf-Type Bellows, washer likes plates of thin metal are welded together, Welding is carried out on both inner and outer circumference of the washer like plate. A welded leaf bellow has more folds per unit length in comparison with formed Bellows. Thus, for the same stroke length, mechanically formed Bellows are two to three times longer than their welded leaf counterparts. Reportedly mechanically formed bellows fail at random spots, while the welded leaf bellow usually fails at or near a weld.
We at `SG’ use only formed bellows, as in spite of being more expensive than welded bellows, their life is many times more than welded bellows.
WHY BELLOW SEALED VALVES?
Leakage from the valve gland is often a problem for the maintenance engineer. Losses incurred due to this type of leakage are considerable. For e.g. (in a 15mm valve) Steam at a pressure of 150 psi escapes through a gland with a clearance of 0.001 in. at a rate of 25 lb/hr. This translates to a loss of approx $ 1.2/8-hr shift $1,100/yr.
A tiny drop of 0.4mm diameter/sec results in a waste of about 200 l/yr of costly oil or solvent. That translates to a loss of approx $1200/yr of a commonly used heating fluid like Dowtherm. And all this for $100 valve!!!
A part from this high cost of energy and material loss, the leakage also results in serious environmental, ecological and containment problems. The leaking fluid can be corrosive, inflammable, explosive or poisonous. In any case, a check needs to be kept on all leaks.
All bellow sealed valve can be a simple solution to all these problems.
TYPES OF BELLOWS
There are two main types of bellows viz “Welded Bellows” and “Formed Bellows”.The latter is made by a relatively newer technology in which a uniform bellow is produced from a thin tube. The shaping of the metal is carried out in a collapsible die under high fluid pressure.
“SG ” prefers Formed Bellows over the Welded Type. The following table will tell you just why :
|Type||Formed Bellow||Welded Bellows|
|Shape of Convolution||Sharp Edge||Round Edge|
|Durability||Reliable durability because bellows are produced from a single piece of metal. Guaranteed 10,000 cycles or more life cycle.||Of doubtful reliability because of a number of welded joints. Plies can be effected by Heat, and thus cycle life is hard to predict.|
|Suitability for use in valve||Because of wide opening between plies of bellows, bellow compression function will not be impaired by bits of foreign matter in fluid.||Wedge-shaped opening between plies will trap foreign matter in fluid, leading to bellow failure.|
|Quality Control||Easy to test for quality, bellow can be produced with uniform quality.||Very difficult to inspect welded spots for quality, hence quality difficult to control.|
|Type Of valves||Bellow Sealed Valves||Piston Valves||Conventional Valves|
|Primary stem Seal||Metallic Bellow||Piston ring||Gland Packing|
|Secondary Stem Seal||Gland Packing||None||None|
|Stem Leakage||Not possible Metallic Bellows are designed to last several thousand cycles||Occurs as soon as the rings(which are made of gasket material) wear out.||Very common. Due to the design conditions, gland leakage occurs with in a few cycles, no matter which make valve or what quality of gland packing|
|Packing Replacement||Not applicable||Rings need to be changed frequently||Need to be replaced very often|
|Replacement Cost||Not applicable||Very high.||High|
|Media Loss||Zero Loss.As the isolation is by metallic bellows, leakage (even in ppm) is not possible||Large in case of Leaks.||Considerable amount through gland leakages. Normally large amount when leakage remain unattended.|
|Equipment Down time||Nil||Very high for replacing rings||Very high for replacing gland packing.|
|Maintenance||Nil||High as rings need to be changed at least once in six months||Very high Gland packing needs to be replaced/repacked often. This cost along with equipment down time, man hours spent etc. is very high.|
|Valve life||High-in years||Low.||Very Low-in months Due to leakage through gland, certain parts erode, making the valve irrepairable after some months. Valve needs to be replaced|
|Safety||Very safe for most of media||Can be used only for limited media like low pressure steam, hot water etc.||Highly unsafe especially when the media is hazardous and poisonous.|
|Cost||Comparatively higher initially but lowest total cost of ownership (TCO) over the years of use.||High compared to the benefits viz a viz problems||Initially low but very high after some years if cost of packing, downtime, man-hours spent, number of valves replaced etc are taken into account.|
Economics ( Economy) Of Using ABellow Seal Valve
For analyzing the cost benefits of a Bellow Seal Valve over a Piston Valve or a conventional Gate / Globe Valve we will shall consider a 15mm valve operating on STEAM at 150 psi, in three types of Valves
Bellow Seal Valve
Standard Globe Valve
CAPITAL COST :- We can estimate the cost per unit as follows
|1. AA) –|
|SR . NO||TYPE OF VALVE||COST PER VALVE IN INR|
|1||Bellow Seal Globe Valve||Rs. 1500.00|
|2||Piston Valve||Rs. 1500.00|
|3||Standard Globe Valve||Rs. 800.00|
Maintenance cost – Assuming that gland leakage will begin within a period of 6 months in a Standard Globe Valve and in a period of 9 months in a Piston Valve, then maintenance costs will be as follows :
|TYPE OF VALVE||COST OF SPARES||COST OF LABOUR||DOWN TIME|
|Piston Valve||Rs.200.00||Rs. 500.00||2 hours|
|Globe Valve||Rs.50.00||Rs.250.00||1 hour|
Assuming cost of spare piston rings at Rs. 200/-per pair of rings.
Assuming a down time of 2 hours x labour cost of Rs. 250/- per hour of
Assuming cost of gland packing at Rs. 50/-
Assuming a downtime of 1 hour x labour cost of Rs. 250/-per hour of maintenance
COST OF DOWNTIME AND ENERGY / MEDIA LOSS COST
We will assume a gland clearance of 1 thou (0.001 inches) for estimation of gland leakage and a cost of Rs. 800/ tonne of Steam. Steam will escape at a rate of 25 lbs/hr through this clearance (as per steam tables).
|TYPE OF VALVE||COST OF DOWNTIME||COST OF LOSS OFMEDIA/ENERGY|
|Bellow Seal Valve||Nil||None|
|Piston Valve||PRICELESS||Rs.6350 per month*|
|Gobe Valve||PRICELESS||Rs.6350 per month*|
|* Rs 6350.00 / 30 Days = 211 per day|
If we assume leakage of 11 kgs / hrs / day =264 kgs of steam loss per day x 30 days =7920 kgs i.e. 7.92 tonne x steam cost of Rs. 800 / Tonne = Rs. 6350/- (US$ 159/-) per month of energy Loss which can be saved !!!!
The cost of down time is priceless and a value as such can not be put on it.
TOTAL COST OF OWNERSHIP
Hence TCO = AA+BB+CC =DD
|VALVE TYPE||AA||BB||CC||DD (TOTAL)|
|Bellow Seal Globe Valve||Rs.1500||NIL||NIL||Rs. 1500.00|
|Piston Valves||Rs.1500||Rs.700||Rs.211/day||Rs.2200 + 211/day|
|Globe Valve||Rs.800||Rs.300||Rs.211/day||Rs.1100 + 211/day|
Total cost of ownership for a BELLOW SEAL VALVE in 1 year would be: Rs 1500/- i.e. US $ 38
Total cost of ownership for a PISTON VALVE in 1 year would be: Rs. 2200+Rs.160/dayx90 days (since we have assumed leakage will begin in 9 months) = Rs.16,600/- i.e. US$ 425
Total cost of ownership for a STANDARD GLOBE VALVE in 1 year would be Rs. 1100 =Rs.160/day x 180 days (since we have assumed leakage will begin in 6 months)= Rs.29, 900/- i.e. US$ 770