Android : Having trouble with firing bullets in unity 3D on Android

Here is my code for shooting bullets

void update()
{
    if(Input.touches.Length > 0)    
    {   
        for (int i = 0; i < Input.touchCount; i++) 
        {
            if(Input.GetTouch(i).phase == TouchPhase.Began)
            {
                if (shootButton.HitTest(Input.GetTouch(i).position) && Time.time > bulletTime) 
                {
                    bulletTime += nextBllet;
                    Ray ray = Camera.main.ViewportPointToRay(new Vector3(0.5f, 0.5f, 0));
                    RaycastHit hit;
                    Vector3 direction;

                    if (Physics.Raycast(ray, out hit))
                    {
                        direction = Vector3.Normalize(hit.point - spawnPoint.position);
                    }
                    else 
                    {
                        direction = spawnPoint.transform.forward;
                    }

                    Quaternion rot = Quaternion.FromToRotation(bulletPrefab.transform.forward, direction);

                    GameObject bull = Instantiate(bulletPrefab, spawnPoint.transform.position, rot) as GameObject;

                    bull.rigidbody.velocity = direction * 60.0f;
                    animator.SetTrigger(shootHash);
                    Destroy(bull.gameObject, 10.0f);
                }   
            }
        }
    }

It will fire a bullet after 2 seconds when I press the shooting button but if I leave the screen alone for couple of seconds and then start tapping the screen, it will keep on firing bullets and then go back to normal. The longer I leave the screen for, the more bullets it will fire if I just tap the screen. The same code works for keyboard and mouse input but I just cant seem to figure out where the problem is.

Can someone please help me?