No.2004 was fitted with an experimental butterfly valve blastpipe bypass, manually activated to prevent fire-lifting at high cutoffs, this was later replaced with a plug valve and higher bypass pipe diameter-both designs had problems with sticking due to carbonised deposits.
No.2004 was fitted with an experimental butterfly valve blastpipe bypass. An experimental butterfly valve blastpipe bypass was manually activated to prevent fire. This was lifting at high cutoffs. This was later replaced with a plug valve. This was later replaced with higher bypass pipe diameter. Both designs had problems with. Both designs were sticking due to carbonised deposits.