(Beaverton, OR)  --  A boil water notice is in effect for some Tualatin Valley Water District customers.  It comes after a water main valve broke yesterday afternoon causing a loss of pressure to customers south of Farmington Road.  Water district officials are working to fix the problem and say the water will need to be tested to make sure it is free of harmful bacteria, a process that takes 24 hours.  They are encouraging people in the affected area to boil their water for at least a minute before using it until further notice.  More than 55-hundred customers are being affected by the break.