Design example Butterfly valve

The service lives of cast iron components in the cooling water circuits of estuarine power stations are often significantly reduced by a number of factors which include graphitic corrosion, localised corrosion due to mixed metals, pitting of surfaces, seizure of sealing faces during outages and brittle fracture. In order to overcome these problems GRP is being considered as an alternative material of construction and there are a number of units now in service. These are large structures with bore diameters up to 1800 mm. The figure below shows details in this case for a component 1524 mm (60 in.) diameter.

Typical design pressures are in the region of 0.5 M Pa (5 bar g). In this design the seal was made of nitrile rubber and was based on the geometry for the previous cast iron designs. The GRP disc thickness of 190 mm (7½ in.) was a compromise between that required for satisfactory disc deflection in the valve-closed position and that required for low hydraulic head loss. The disc was an all-woven roving GRP construction, except for the seal clamping ring and bolts, the stub shafts and their housings, all of which were constructed from stainless steel. A woven roving and isophthalic resin system was chosen, because of its relatively high modulus and proven durability and as it is a reasonably economic form of composite material. The body was of a 'wafer' design, being clamped by long through-bolts between the adjacent pipe flanges. This design avoids the conventional flanges in the body, which could be subject to unknown system loads, and also provides a sufficient material thickness to limit the body's bending deflections due to pressure. The construction consisted of alternating layers of woven rovings and CSM, this type of construction having a higher shear strength than the all-woven roving design, but a lower modulus. Again an isophthalic polyester resin system was used. The surface mating with the rubber seal on the disc was the moulded gel coat, which therefore avoided the corrosion or seizing mechanism which occurs in the cast iron valve. The flange mating with . the actuator gearbox must be capable of withstanding the motor stall torque of 27 kN m (20 000 lb/ft) and the bending moments due to the gearbox and actuator dead weight. Lifting lugs were omitted from the design as it was considered that the relatively low weight of the valve would enable it to be installed by a fork lift truck, rather than requiring a crane. The valve geometry was designed to material properties previously found to be typical for the two laminate types, based on the chosen resin and glass cloths. The valve body was constructed with alternating layers of woven roving and chopped strand mat. For this material, a modulus of 10 GPa and a strength of 145 N/mm² were assumed. The valve blade on the other hand was constructed from all-woven roving, because of its higher modulus, which was expected to be 14.5 GPa. Its anticipated strength was 215 N/mm². The Table shows details of material properties measured.

The stress analysis of the valve disc assuming constant thickness and homogeneous material properties is given below. The equations assume a constant thickness disc, supported at two diametrically opposite points on the perimeter and under constant pressure loading. These equations were used to give the theoretical bending moments and deflections for the butterfly-valve disc. The relationships derived were as follows:

where M_r, and M_t are radial and tangential bending moments per unit length respectively, p the water pressure on disc, n the Poisson's ratio, a the disc radius, r, q the polar co-ordinates, and r is the ratio r/a.

The disc transverse deflection, w, is given by the expression:

where

It was found that, on areas of the disc away from stress concentrations, there was agreement between the measured and theoretical stresses to within ± 14%. Also, there was agreement between the measured and theoretical disc deflections to within ± 5% shown below.

Disc deflections at 0.5 MPa (60 psi)

Thus, even though the theoretical analysis was based on homogeneous material properties, and a constant thickness disc, it may be used with confidence at the design stage for thickness calculations. Valves of this size are now in service and during the reported inspection three years after installation the components were found to be fully satisfactory. At the time of manufacture the cost GRP alternative was found to be marginally more expensive (16%) than its traditional counterpart, coated cast iron, but these were found to be difficult to source. The other option using a corrosion-resistant metallic design would have been approximately 30% more expensive than GRP.